vdsm.spec.in: with_gluster

Option to manage Gluster sub-package.

Change-Id: Ifebbd38d3d225c22f1f39a2f8758591066824e76
Signed-off-by: Douglas Schilling Landgraf <dougsland@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/13542
Reviewed-by: Federico Simoncelli <fsimonce@redhat.com>