Keeping gluster modules when with_gluster set to 0

Currently the code imports and uses gluster modules. Setting
with_gluster=0 brakes the code. With this patch with_gluster=0 will
only drop the vdsm-gluster rpm creation.

Change-Id: I1cdbd8b399652bf0d306308e2d4cfc0014be689c
Signed-off-by: Yaniv Bronhaim <ybronhei@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/21477
Reviewed-by: Dan Kenigsberg <danken@redhat.com>