Returns the agency running the agent with agent_id or None.

def find_agency(self, agent_id):
        """
        Returns the agency running the agent with agent_id or None.
        """

        def has_agent(agency):
            for agent in agency._agents:
                if agent._descriptor.doc_id == agent_id:
                    return True
            return False

        matching = filter(has_agent, self._agencies)
        if len(matching) > 0:
            self.debug('Find agency returns %dth agency.',
                       self._agencies.index(matching[0]))
            return matching[0]