add metadata=0.90 to mdadm --create

During installation of sl6 machines with software raid sometimes they spit errors when installing grub, and then fail to start after reboot. Adding metadata=0.90 appears to fix this. Drawbacks can be found at https://raid.wiki.kernel.org/index.php/RAID_superblock_formats , none seems critical though. Perhaps in the future add functionality to change mdadm invocations based on OS version?