Corrected GlusterFS driver info on snapshots

The current docs state that the GlusterFS driver does not support
snapshots and clone. This is no longer true.

This patch updates the documentation accordingly.

Change-Id: I0a6f5ba84e846c117a7f425e41b4bf28dfe552e6
Partial-Bug: #1279185
Merge "Corrected GlusterFS driver info on snapshots"