Shelve/unshelve an instance

This adds the compute methods for shelving and unshelving.  Shelving
will power down the instance, and snapshot if not volume backed.
Unshelving will restore the instance, which may involve re-scheduling
and rebuilding it from a snapshot.

Part of bp shelve-instance

Co-Author: Dan Smith <danms@us.ibm.com> (Instance objects)
Change-Id: I908474915c5325e120a64af9b9af13fcbce1f01c
Merge "Shelve/unshelve an instance"