Remove instance exists check from rebuild_instance

This existence check seems completely bogus - surely the instance must
exist in order to rebuild it?

This issue wasn't papered over because we weren't running evacuate with
a valid compute node hostname so the rebuild_instance() gets sent to a
bogus queue.

That call on a bogus queue will fail if we start timing out call()s in
the fake RPC driver when there are no consumers for a topic.

blueprint: oslo-messaging
Change-Id: Iffebf782ab3fbf3f25cceb068ef24813808f5045
Merge "Remove instance exists check from rebuild_instance"