returns a authentication handler.
    Get user & password from grains, if are not set default to
    modules.config.option

    If user & pass are missing return False

def _auth(uri):
    '''
    returns a authentication handler.
    Get user & password from grains, if are not set default to
    modules.config.option

    If user & pass are missing return False
    '''

    user, password = _get_credentials()
    if user is False or password is False:
        return False

    basic = _HTTPBasicAuthHandler()
    basic.add_password(realm='Tomcat Manager Application', uri=uri,
                       user=user, passwd=password)
    digest = _HTTPDigestAuthHandler()
    digest.add_password(realm='Tomcat Manager Application', uri=uri,
                        user=user, passwd=password)
    return _build_opener(basic, digest)