GlusterFS: Ensure Cinder can write to shares

Ensure the Cinder user can write to the GlusterFS share.  This
is required for snapshot functionality, and means the admin
does not have to set this permission manually.

Conflicts:
	cinder/tests/test_glusterfs.py

Closes-Bug: #1236966
Change-Id: I4a9ea40df9681ca6931ad6b390aa21b09d6cfec9
(cherry picked from commit 371fa540600b20b97eae389e1f976145866cadae)
GlusterFS: Complete snapshot_delete when info doesn't exist

The snapshot_delete operation will fail if the snapshot info file
doesn't contain a record for the snapshot, or does not exist.
This happens in cases such as when snapshot_create fails to commit
anything to disk.

The driver should allow the manager to delete the snapshot
in this case, as there is no action required for the driver
to delete anything.

Closes-Bug: #1252864

(cherry picked from commit d8a11168c908fe6c6a07fbb30a5bc88a6df6e939)

Change-Id: I8686a1be09dbb7984072538bff6c026bb84eeb52
Merge changes I8686a1be,I4a9ea40d into stable/havana

* changes:
  GlusterFS: Complete snapshot_delete when info doesn't exist
  GlusterFS: Ensure Cinder can write to shares