Make delete_instance_metadata() use objects

This adds an optimized Instance.delete_metadata_key() method
and makes compute_api use it instead of calling directly to the
database.

This changes a test that previously verified that the notification
was missing instance.metadata (because of the rpc calling convention)
since we can now include the updated metadata in the notification
easily.

Related to blueprint compute-manager-objects

Change-Id: I827cb98d004e7a612b839dd5a8bbe26cc91c692d
Merge "Make delete_instance_metadata() use objects"