Fix disk detection

Nodes with multiple devices need a random disk identifier written
on the disk so that the BIOS can detect boot order properly.
fdisk does that. parted nukes the identifer. In addition the
parted code only handled /dev/sd? devices, not /dev/cciss or
/dev/hdX or whatever else we support right now.
Merge pull request #186 from dirkmueller/fix_disk_detection

Fix disk detection