Revert "gluster prepareImage: return gluster-specific information"

This reverts commit 51081b81de8cda48e0060e which is universally known to
be quite ugly, and is also unhelpful since we do not support
gluster-based VM with snapshots.

Change-Id: I9b90be682593183434012ddfd5578af38bfa2e0b
Signed-off-by: Dan Kenigsberg <danken@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/21508
Reviewed-by: Federico Simoncelli <fsimonce@redhat.com>