Stub out instance_update() in build instance tests

Change Ife6e5aef4fda46f74e2db7be1a09b210be397152 introduced an
instance_claim() which invokes an instance_update() RPC on conductor.
However, during tests no conductor is running and this RPC would have
happily returned None. If we if we time out call()s in the fake RPC
driver when there are no consumers for a topic, then these tests will
fail.

To avoid this, simply mock out the instance_update() conductor calls.

blueprint: oslo-messaging
Change-Id: Iea010d92eadaf15b3a9e339589170a26234b3142
Merge "Stub out instance_update() in build instance tests"