Use same constants for configuring and check configure of sanlock

Both DISKIMAGE_GROUP and QEMU_PROCESS_GROUP points to 'qemu'. It's
confusing to use both one for configure and one for verification.
The patch uses the same constants and check also if VDSM_GROUP is also
set properly as required by sanlock process.

Change-Id: I424fdf30e0c5377846976505633d674f21ee7438
Signed-off-by: Yaniv Bronhaim <ybronhei@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/24011
Reviewed-by: Dan Kenigsberg <danken@redhat.com>