LU-2374 mkfs: Make order of --index and --ost irrelevant

The order of --index and --ost should not prevent mkfs.lustre to
work. So reordered the checks which ensure that  mkfs with index is
done for ost and mdt only.

Signed-off-by: Girish Shilamkar <gshilamkar@ddn.com>
Change-Id: I228f6c9189aa85a5565393523d39e8e27e0eaddc