fence_scsi: write devices to tmp file on unfence
    
    When unfencing occurs (action=on), write list of devices that were
    successfully registered to /var/run/cluster/fence_scsi.dev file. This
    file will be used by the optional watchdog script. Note that the devices
    are not added to the tmp file until the verification step.
    
    Signed-off-by: Ryan O'Hara <rohara@redhat.com>