Offline snapshots for GlusterFS volumes

Offline snapshots for GlusterFS volumes

Offline snapshots for the GlusterFS driver, based
on QCOW2 snapshotting.

This introduces the option to have volumes stored as
qcow2 files rather than raw files. (glusterfs_qcow2_volumes)

Bump driver version to 1.1.0.

This patch requires Nova to support loading qcow2 disks:
https://review.openstack.org/#/c/38321/

Online snapshot support (via Nova) is added in a
subsequent patch submitted in this series.

This patch also enables volume cloning for GlusterFS volumes.

Implements blueprint qemu-assisted-snapshots

Change-Id: I783a802574b835b495e343f555f7ecf9b46981dd