Print information about the slave agencies.

def list_slaves(self):
        '''Print information about the slave agencies.'''
        resp = []
        for slave_id, slave in self._broker.slaves.iteritems():
            resp += ["#### Slave %s ####" % slave_id]
            table = yield slave.callRemote('list_agents')
            resp += [table]
            resp += []
        defer.returnValue("\n".join(resp))