Do not use qemu-img --backing-chain or --output=json

Initial code for GlusterFS snapshot support in Havana
used qemu-img arguments which are too new to run in many
environments.  Run qemu-img using only arguments which
will exist on older platforms.

Change-Id: I5155dac492da67a951ede978c2c46a54c239eb04
Convert to image_utils for qemu-img info and resize

Code added for GlusterFS snapshot support in Havana included
new code to parse qemu-img output.  This removes this code
and uses image_utils qemu_img_info instead.

Improve test coverage with tests for cloning, create from
snapshot, and initialize_connection.

Change-Id: I8f1811b400c06edb3cd7416c52aa297921841e54
GlusterFS: Use correct source file (copy_volume_from_snapshot)

The GlusterFS driver's copy_volume_from_snapshot was previously
not parsing the snapshot information to determine which backing
file to copy from, instead copying from the active file at all
times.  This would result in the wrong data being supplied when
a volume is cloned from a snapshot.

Closes-Bug: 1222907

Change-Id: Ib829ca1a6812b61845f3b2eb9c5507779fa6ec15