LU-6662 utils: allow overriding default mountopts

LU-6662 utils: allow overriding default mountopts

during formatting the device for Lustre, there are two kinds of
mount options to be used by "mkfs.lustre", which is default and
wanted options. the wanted options is mandatory but the default
should only be used if user doesn't specify it.

Signed-off-by: Hongchao Zhang <hongchao.zhang@intel.com>
Change-Id: I7d8093082455e0a08eb40df63dbc938d6dd8a2a6
Reviewed-on: http://review.whamcloud.com/15870
Tested-by: Jenkins
Reviewed-by: Nathaniel Clark <nathaniel.l.clark@intel.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>