Make compute_api use InstanceAction object

This makes the compute_api instance action code use objects instead
of direct-to-database calls. This should open the door for deprecating
some conductor proxies for compute_api that help compute manager
avoid the database hit required for the instance action to complete.

Related to blueprint compute-api-objects

Change-Id: If622d8079ca2897b0d5094e9341e29cde7c1ff59
Merge "Make compute_api use InstanceAction object"