GlusterFS: Support over subscription in thin provisioning

GlusterFS: Support over subscription in thin provisioning

This patch adds "over subscription in thin provisioning" support to
GlusterFS driver.

The following changes are proposed:
* GlusterFS driver reports the following new capacities and ratios:
  * provisioned_capacity
  * max_over_subscription_ratio
  * thin_provisioning_support (True or False)
  * thick_provisioning_support (True or False)

Co-Authored-By: Deepak C Shetty <deepakcs@redhat.com>
Implements: blueprint glusterfs-over-subscription-thin-provisioning
Change-Id: I7fbbe989c865dce42102b63f7d52a8edb63e7a9e