Fix _controller instantiation in agent network mode connect=True for osbrain backend

[bangxiangyong]

In network.py: when osbrain backend is used, there is a crucial feature for client to gain access to the agent network nameserver by setting connect=True: this connects to an existing agent network (possibly situated in a remote server) instead of starting one.
However, one missing instantiation is the self._controller, which should be fixed by adding this line immediately after line 591:
self._controller = self.ns.proxy("AgentController"), which otherwise will remain as None.