LU-2022 utils: Mount MGS lacking index

LU-2022 utils: Mount MGS lacking index

The MGT is the only device that can't be formatted with a
index but mount.lustre expects all devices to be formated
with a index. This patch allows the MGT to be the exception.

Signed-off-by: James Simmons <uja.ornl@gmail.com>
Change-Id: Icea4e18173e9c670302a795bd53b2769b55fc38f
Reviewed-on: http://review.whamcloud.com/4084
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Keith Mannthey <keith@whamcloud.com>
Reviewed-by: Alex Zhuravlev <bzzz@whamcloud.com>