Authenticates a user via the Keystone Identity API.

def authenticate(self, auth_url=None, **kwargs):
        """Authenticates a user via the Keystone Identity API."""
        LOG.debug('Beginning user authentication')

        if not auth_url:
            auth_url = settings.OPENSTACK_KEYSTONE_URL

        auth_url, url_fixed = utils.fix_auth_url_version_prefix(auth_url)
        if url_fixed:
            LOG.warning("The OPENSTACK_KEYSTONE_URL setting points to a v2.0 "
                        "Keystone endpoint, but v3 is specified as the API "
                        "version to use by Horizon. Using v3 endpoint for "
                        "authentication.")

        plugin, unscoped_auth = self._get_auth_backend(auth_url, **kwargs)

        # the recent project id a user might have set in a cookie
        recent_project = None
        request = kwargs.get('request')
        if request:
            # Grab recent_project found in the cookie, try to scope
            # to the last project used.
            recent_project = request.COOKIES.get('recent_project')
        unscoped_auth_ref = plugin.get_access_info(unscoped_auth)

        # Check expiry for our unscoped auth ref.
        self.check_auth_expiry(unscoped_auth_ref)

        domain_name = kwargs.get('user_domain_name', None)
        domain_auth, domain_auth_ref = plugin.get_domain_scoped_auth(
            unscoped_auth, unscoped_auth_ref, domain_name)
        scoped_auth, scoped_auth_ref = plugin.get_project_scoped_auth(
            unscoped_auth, unscoped_auth_ref, recent_project=recent_project)

        # Abort if there are no projects for this user and a valid domain
        # token has not been obtained
        #
        # The valid use cases for a user login are:
        #    Keystone v2: user must have a role on a project and be able
        #                 to obtain a project scoped token
        #    Keystone v3: 1) user can obtain a domain scoped token (user
        #                    has a role on the domain they authenticated to),
        #                    only, no roles on a project
        #                 2) user can obtain a domain scoped token and has
        #                    a role on a project in the domain they
        #                    authenticated to (and can obtain a project scoped
        #                    token)
        #                 3) user cannot obtain a domain scoped token, but can
        #                    obtain a project scoped token
        if not scoped_auth_ref and domain_auth_ref:
            # if the user can't obtain a project scoped token, set the scoped
            # token to be the domain token, if valid
            scoped_auth = domain_auth
            scoped_auth_ref = domain_auth_ref
        elif not scoped_auth_ref and not domain_auth_ref:
            msg = _('You are not authorized for any projects.')
            if utils.get_keystone_version() >= 3:
                msg = _('You are not authorized for any projects or domains.')
            raise exceptions.KeystoneAuthException(msg)

        # Check expiry for our new scoped token.
        self.check_auth_expiry(scoped_auth_ref)

        # We want to try to use the same region we just logged into
        # which may or may not be the default depending upon the order
        # keystone uses
        region_name = None
        id_endpoints = scoped_auth_ref.service_catalog.\
            get_endpoints(service_type='identity')
        for id_endpoint in [cat for cat in id_endpoints['identity']]:
            if auth_url in id_endpoint.values():
                region_name = id_endpoint['region']
                break

        interface = getattr(settings, 'OPENSTACK_ENDPOINT_TYPE', 'public')

        endpoint, url_fixed = utils.fix_auth_url_version_prefix(
            scoped_auth_ref.service_catalog.url_for(
                service_type='identity',
                interface=interface,
                region_name=region_name))
        if url_fixed:
            LOG.warning("The Keystone URL in service catalog points to a v2.0 "
                        "Keystone endpoint, but v3 is specified as the API "
                        "version to use by Horizon. Using v3 endpoint for "
                        "authentication.")

        # If we made it here we succeeded. Create our User!
        unscoped_token = unscoped_auth_ref.auth_token

        user = auth_user.create_user_from_token(
            request,
            auth_user.Token(scoped_auth_ref, unscoped_token=unscoped_token),
            endpoint,
            services_region=region_name)

        if request is not None:
            # if no k2k providers exist then the function returns quickly
            utils.store_initial_k2k_session(auth_url, request, scoped_auth_ref,
                                            unscoped_auth_ref)
            request.session['unscoped_token'] = unscoped_token
            if domain_auth_ref:
                # check django session engine, if using cookies, this will not
                # work, as it will overflow the cookie so don't add domain
                # scoped token to the session and put error in the log
                if utils.using_cookie_backed_sessions():
                    LOG.error('Using signed cookies as SESSION_ENGINE with '
                              'OPENSTACK_KEYSTONE_MULTIDOMAIN_SUPPORT is '
                              'enabled. This disables the ability to '
                              'perform identity operations due to cookie size '
                              'constraints.')
                else:
                    request.session['domain_token'] = domain_auth_ref

            request.user = user
            timeout = getattr(settings, "SESSION_TIMEOUT", 3600)
            token_life = user.token.expires - datetime.datetime.now(pytz.utc)
            session_time = min(timeout, int(token_life.total_seconds()))
            request.session.set_expiry(session_time)

            keystone_client_class = utils.get_keystone_client().Client
            session = utils.get_session()
            scoped_client = keystone_client_class(session=session,
                                                  auth=scoped_auth)

            # Support client caching to save on auth calls.
            setattr(request, KEYSTONE_CLIENT_ATTR, scoped_client)

        LOG.debug('Authentication completed.')
        return user