Add support for enabling consistency group api

I think the feature here is to add support for enabling the consistency group API, rather than to provide direct management of the policy.json (which I remain a -1 on).

Marking as Triaged/Wishlist for now.

I've commented on https://review.openstack.org/#/c/525873 to this effect; enabling support for the CG API should be driven based on the storage backends that form part of the deployment, rather than being a blind configuration option that the user has to enable.

For example, CG's don't have context for our default storage backend (ceph) so it makes no sense to even allow a user to attempt to enable them in this use-case; if say we had a cinder-netapp subordinate charm it *would* make sense to expose CG support on the cinder-netapp charm and use its relation to cinder to instruct the cinder charm to enable the required policy changes.

Change abandoned by Seyeong Kim (<email address hidden>) on branch: master
Review: https://review.openstack.org/525873