LU-3768 tunefs: make tunefs.lustre work correctly for MGS

LU-3768 tunefs: make tunefs.lustre work correctly for MGS

Since LDD_F_NEED_INDEX flag is unset for a stand-alone MGS during
mkfs.lustre, when running tunefs.lustre on that MGS, it always says
"'----index' only valid for MDT,OST". To fix that, this patch is to
- print that error message only for mkfs.lustre
- fix the typo '----index' in the error message.

Signed-off-by: Liu Ying <emoly.liu@intel.com>
Change-Id: I4fa00dc554521c03671602ff9b2bbaca02c6a581
Reviewed-on: http://review.whamcloud.com/7353
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Bobi Jam <bobijam@gmail.com>
Reviewed-by: Niu Yawei <yawei.niu@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>