Stubout deallocate_for_instance() in compute tests

Otherwise it gets called during delete() and 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: I14516d43b2850c554cb997b3d41d20db04ff7a9b
Merge "Stubout deallocate_for_instance() in compute tests"