Move block_device_mapping get operations to conductor

This patch moves the block_device_mapping_get_by_instance()
uses from compute/manager to the conductor. This also includes
some nasty refactoring in some of compute's helper methods to
pass around instances instead of just uuids so that we can
properly pass the former to the conductor.

Related to blueprint no-db-compute-manager

Change-Id: Ib5223274d21096a9e47b3420c3de801b978b253d
Merge "Move block_device_mapping get operations to conductor"