MRP-2046 utils: "-G <value>" can be passed trough mkfs options

mkfs.lustre util checks if parametrs already contain "flex_bg"
option (number of block groups that will be packed together
to create a larger virtual block group)  and adds this option
if it doesn't exists. But second parameter "-G" (number of block
groups that will be packed together to create a larger virtual
block group) can be added twice: one passed trough mkfs options
and another one - default value. In this case this parameter
is not changed actually and default value is applied.

This patch adds extra check. Default option "-G" is added
only if no "-G" option passed through mkfs options.

Change-Id: I8939776c7236b329dd7bff54879b7a8b53e85763
Signed-off-by: Artem Blagodarenko <artem_blagodarenko@xyratex.com>