This patch addresses the following:
* Modify Consistency Group
* Add CLI to update a consistency group, i.e., changing name and
description, adding existing volumes to CG and removing volumes
from CG.
cinder consisgroup-update --name <name> --description <description>
--add-volumes <uuid1,uuid2,...> --remove-volumes <uuid5,uuid6,...>
Implements: blueprint consistency-groups-kilo-update
Change-Id: I76317dc006c1f80e2e6c83218e9566f1d37d935e
f12aab8035