Finds an appropriate agent entry for the given location.
        :return Agent

def _find_agent(self, location):
        ''' Finds an appropriate agent entry for the given location.
        :return Agent
        '''
        for a in self._auth_info.agents:
            # Don't worry about trailing slashes
            if a.url.rstrip('/') == location.rstrip('/'):
                return a
        raise httpbakery.InteractionMethodNotFound(
            'cannot find username for discharge location {}'.format(location))