make wipe_metadata faster

Get the size of the drive in logical sectors (512 bytes), then wipe the first 10 and last 20 sectors.

We have to use fdisk because parted in SL5 will refuse to display the size of an unlabeled drive.

The first 10 and last 20 sectors were chosen after reading http://kezhong.wordpress.com/2011/06/14/how-to-remove-bios-raid-metadata-from-disk-on-fedora/. They have been tested with RAL diskservers. They should be enough to wipe gpt partition information and raid metadata.