LU-3559 utils: mgs has no index

LU-3559 utils: mgs has no index

An MGS formatted with Lustre 2.1 does not have any index.
So mount.lustre must not require an index when starting
an MGS.

Signed-off-by: Sebastien Buisson <sebastien.buisson@bull.net>
Change-Id: I12c1a2511509c4d34ef60a0ed937dda9e5fba4e4
Reviewed-on: http://review.whamcloud.com/6904
Tested-by: Hudson
Reviewed-by: Faccini Bruno <bruno.faccini@intel.com>
Reviewed-by: James Simmons <uja.ornl@gmail.com>
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Li Wei <wei.g.li@intel.com>
Reviewed-by: Alex Zhuravlev <alexey.zhuravlev@intel.com>