LU-12514 utils: try lustre_tgt filesystem for mount

LU-12514 utils: try lustre_tgt filesystem for mount

Now that Lustre supports a separate file system for the server
targets we can update mount.lustre to try "lustre_tgt" for the
server backend. For backwards compatibility if using "lustre_tgt"
fails try the original "lustre" file system type.

Change-Id: Ica08ff2abc3ad06c78d6d435db7e2fa3897e037e
Signed-off-by: James Simmons <jsimmons@infradead.org>
Reviewed-on: https://review.whamcloud.com/38799
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Yang Sheng <ys@whamcloud.com>