vdsm.spec.in: with_gluster

Option to manage Gluster sub-package.

Change-Id: I925c1b12f0447798eaae6da72d726bc007511e1c
Signed-off-by: Douglas Schilling Landgraf <dougsland@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/13731
Reviewed-by: Federico Simoncelli <fsimonce@redhat.com>