GlusterFS: Use image_utils for qemu-img calls

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.

Add test to image_utils for parsing a different style of
qemu-img info output.

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

Closes-Bug: #1224030
Change-Id: I8f1811b400c06edb3cd7416c52aa297921841e54
remove duplication, clean up stuff
GlusterFS: Copy snap from correct source file

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.

Add a test for the copy_volume_from_snapshot method.

Closes-Bug: #1222907
Change-Id: Ib829ca1a6812b61845f3b2eb9c5507779fa6ec15