GlusterFS: Lock on a per-volume basis

This allows operations that do not conflict with each
other (i.e. are on different volumes) to run concurrently.
The prior locking scheme was too coarse and essentially
made the driver single-threaded.

Change-Id: Ib048c21f66926cb96ae02ced4bd16a752d52b301
Scality SOFS: implement volume backup and restore

Add support in Scality SOFS driver for volume backup interface
to backup or restore a volume using backup service
specified in cinder.conf.

Change-Id: I6b4b132c7aec9d0e9e558fd30036cf95d80698d1
Merge "Scality SOFS: implement volume backup and restore"
Merge "GlusterFS: Lock on a per-volume basis"