API for shelving

Adds new 'shelve', 'shelveOffload'/'shelve_offload'(V3), and 'unshelve'
actions to the API.  Exposes the functionality already provided in the
compute api.

Part of bp shelve-instance

Co-author: Dan Smith <danms@us.ibm.com> (Instance objects)
Change-Id: Idd485b591730c6ac025ee57a1242afdd02191b2f
Merge "API for shelving"