Stub out net API sooner in servers API test

Currently we're stubbing out the network API after the controller has
already cached a pristine copy of it.

Without this, validate_networks() fails if we time out call()s in the
fake RPC driver when there are no consumers for a topic.

blueprint: oslo-messaging
Change-Id: Ia777f964d56678cabb62c1c1dc5db5684ef5dbc0
Merge "Stub out net API sooner in servers API test"