fix extract os info

Change-Id: I04b9f9fab2505620833c66e0c050f48577e80ac7
Reviewed-on: http://review.couchbase.org/33945
Tested-by: Thuan Nguyen <soccon@gmail.com>
Reviewed-by: Ketaki Gangal <ketakigangal@gmail.com>
Reviewed-by: Thuan Nguyen <soccon@gmail.com>