Stubout reserve_block_device_name() in test

Otherwise it gets called during attach_volume() 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: I81a8e7fc3c10b6f1ddbec7b1c4ddbbfc9bb45934
Merge "Stubout reserve_block_device_name() in test"