List agents hosted by the agency.

def list_agents(self):
        '''List agents hosted by the agency.'''
        t = text_helper.Table(fields=("Agent ID", "Agent class", "State"),
                              lengths=(40, 25, 15))

        return t.render((a._descriptor.doc_id, a.log_category,
                         a._get_machine_state().name)
                        for a in self._agents)