Fix live migration test for no scheduler running

If we time out call()s in the fake RPC driver when there are no
consumers for a topic, the live_migrate() test fails when we attempt
to call() out to scheduler.

Mock up the live_migration() rpcapi and verify it gets called.

blueprint: oslo-messaging
Change-Id: Id06a4ff29a0bb4d2caee17576a0ff27bfab4cd87
Merge "Fix live migration test for no scheduler running"