tells the host agent running in this agency to restart the agent.

def restart_agent(self, agent_id, **kwargs):
        '''tells the host agent running in this agency to restart the agent.'''
        host_medium = self.get_medium('host_agent')
        agent = host_medium.get_agent()
        d = host_medium.get_document(agent_id)
        # This is done like this on purpose, we want to ensure that document
        # exists before passing it to the agent (even though he would handle
        # this himself).
        d.addCallback(
            lambda desc: agent.start_agent(desc.doc_id, **kwargs))
        return d