LU-4227 utils: mgs option is required to format a MDT/OST

LU-4227 utils: mgs option is required to format a MDT/OST

When running mkfs.lustre to format a MDT, --mgs or --mgsnode option
is required; while to format a OST, --mgsnode option is required.
conf-sanity.sh test_79 is added to verify the fix.

Signed-off-by: Emoly Liu <emoly.liu@intel.com>
Change-Id: Ia65b382aad2bc31496f3da62f15349ec38990ea0
Reviewed-on: http://review.whamcloud.com/8387
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Niu Yawei <yawei.niu@intel.com>
Reviewed-by: Faccini Bruno <bruno.faccini@intel.com>