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