Fix rpc calls on pre/post live migration tests

If we time out call()s in the fake RPC driver when there are no
consumers for a topic, lots of tests fail when we attempt to call()
out to the compute service to do pre_live_migration().

Also use the same method to mock out some post_live_migration() calls
which are currently done a pretty awkward way.

blueprint: oslo-messaging
Change-Id: Ifb1675cfbc3c6a325f0a3d2d130268561342001e
Merge "Fix rpc calls on pre/post live migration tests"