Make compute_api migrate/resize paths use instance objects

This converts the migrate/resize/live_migrate paths to use instance
objects.  This also syncs up cells rpcapi and compute task rpcapi so
we can ditch the special cells code in compute/cells_api.py.

Related to blueprint compute-api-objects

Change-Id: I977a5edb8a5313b2a8827796ff8c2bc081f9d63f
Merge "Make compute_api migrate/resize paths use instance objects"