Fix network manager tests to use correct network host

Without this, if we time out call()s in the fake RPC driver when there
are no consumers for a topic, then the test fails when it tries to call
_disassociate_floating_ip() on a non-existent host.

blueprint: oslo-messaging
Change-Id: I99c9c75c07145a60b43701826fb680a346ec955f
Merge "Fix network manager tests to use correct network host"