Cells: Pass instance objects to update/delete_instance_metadata

When nova-cells processes a 'run_compute_api_method' message it pulls
the instance from the local db and calls the appropriate compute api
method with a sqlalchemy model.  Since the update_instance_metadata and
delete_instance_metadata methods in the compute api require objects
these methods are added to the list of methods that will receive an
object.

Change-Id: Iaa5ba6878f0162e2fab8dd3e4b87ea8dd3d527f6
Closes-bug: #1290975
Merge "Cells: Pass instance objects to update/delete_instance_metadata"