Run as part of the handshake.
        @param broker: Remote reference to the broker object

def become_slave(self, broker):
        '''
        Run as part of the handshake.
        @param broker: Remote reference to the broker object
        '''
        self._set_state(BrokerRole.slave)
        self._master = broker
        d = defer.succeed(None)
        if callable(self.on_slave_cb):
            d.addCallback(defer.drop_param, self.on_slave_cb)

        d.addCallback(defer.drop_param, self._master.callRemote,
                      'handshake', self, self.agency, self.agency.agency_id,
                      self.is_standalone())
        d.addCallback(defer.inject_param, 1, self.update_state,
                      'reset_locally')

        for medium in self.agency.iter_agents():
            d.addCallback(defer.drop_param, self.register_agent, medium)

        return d