Add provisioned_capacity

Add provisioned_capacity

This change is needed by the over subscription patch:
https://review.openstack.org/#/c/142171/

This patch makes the following change:
Add 'provisioned_capacity' to Cinder base driver, Cinder reference
driver (LVM), and scheduler host_manager.

provisioned_capacity is the apparent allocated space indicating how
much capacity has been provisioned.

Example: User A created 2x10G volumes in Cinder from backend A, and
user B created 3x10G volumes from backend A directly, without using
Cinder. Assume those are all the volumes provisioned on backend A.
The total provisioned_capacity will be 50G and that is what the driver
should be reporting.

Change-Id: I26035a8b591309a922efa517e50c68eb57ef6d2d
Partial-Implements: blueprint over-subscription-in-thin-provisioning