LU-2896 mgs: fix handling non IR targets.

non IR targets don't know about extra command as have single
"register target" so we need to treat - none command
as register new target

Xyratex-bug-id: MRP-880
Signed-off-by: Alexey Lyashkov <alexey_lyashkov@xyratex.com>
Change-Id: I22a8fd034772c9355d2d56a166fdf3766edec719
Reviewed-on: http://review.whamcloud.com/5574
Tested-by: Hudson
Reviewed-by: Jinshan Xiong <jinshan.xiong@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Tested-by: Maloo <whamcloud.maloo@gmail.com>