LU-7299 utils: allow mkfs.lustre --index to specify in hex/dec

LU-7299 utils: allow mkfs.lustre --index to specify in hex/dec

The mkfs.lustre --index argument should be able to handle hex
index values as well as decimal values, especially since the
OSTxxxx identifiers are printed in hexadecimal as well.

Signed-off-by: Thomas Stibor <t.stibor@gsi.de>
Change-Id: I9f7564e3d674353fbebef18bde1598c01bb5bb2c
Reviewed-on: http://review.whamcloud.com/16831
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>