Implement get_volume_stats for GlusterFS driver

Implement get_volume_stats(), so that the GlusterFS driver
will work correctly with the scheduler.

Fixes bug: 1164038

Change-Id: Ib4db8bad897212227b39704e2e6cb5f07f2bb70e
(cherry picked from commit d59094f316f0c41aaa7be63ee4e25e2dea4fa0a9)