This change enables the attach and detach volume operations for
instances which are in shelve and shelved_offloaded state.
The code to manage the actual attach and detach is implemented by this
change: https://review.openstack.org/259528
New tempest tests are going to be written for testing the new feature.
APIImpact
DocImpact by this change we are allowing operations which were not
allowed before, we need to track this change in the documentation.
Partially implements blueprint: volume-ops-when-shelved
Change-Id: I43b67a50d998d43a6ff78c917093c513231b6ff2