gluster: fix nasty 'glusterfs not installed' vdsm.log warnings

Change-Id: Ia701ddbce65bc4f48a58dd40f83bb9bbb0a9bf46
Signed-off-by: Timothy Asir <tjeyasin@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/13428
Reviewed-by: Aravinda VK <avishwan@redhat.com>
Tested-by: Aravinda VK <avishwan@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>