Make admin API state resets use Instance.save()

This removes the 'update_state' compute API method in favor of having
the admin API use the Instance object's save() method.

This also removes the non-unity cells test for resetting state.  Cells
behavior is tested via tests on Instance.save().

Related to blueprint compute-api-objects

Change-Id: I28d1ca9d711679cc55375774e879870f457a5b0b
Merge "Make admin API state resets use Instance.save()"