core: Prevent usage of shareable disks on Gluster domains

To avoid risk of split brain on Gluster domains this patch prevents the
creation of shareable disks on Gluster domains as well as the update of
existing disks to shareable.
Also on creation of a new Gluster domain via webadmin there a message that
tells to use Quorum on the Gluster server to ensure data integrity

Change-Id: I6d741c93d32fa568c1d1f10429cb325a9b01c359
Bug-Url: https://bugzilla.redhat.com/1024654
Signed-off-by: Tal Nisan <tnisan@redhat.com>