simplify metadata wiping

Based on http://kezhong.wordpress.com/2011/06/14/how-to-remove-bios-raid-metadata-from-disk-on-fedora/

using fdisk -s to get the sectors and then delete the first 2 and the last 20 sectors. Simpler mathematics (SIZE/1024 was erroring too often with large drives), tested that wipes gpt metadata successfully. Cannot use parted yet as version in SL5 (and possibly 6) refused to work on unlabeled drives (ie new drives)