Only grab the first disk when grabbing first disk selected

It always grabbed all unclaimed disks before.
Clean partition (and partition table) to let pvcreate succeed

If there is other stuff remaining on the partition for cinder,
pvcreate will fail. So when the disk is grabbed,clean it, and
then run pvcreate. Fixes
https://bugzilla.novell.com/835942
Merge pull request #64 from dirkmueller/fix_first_grabbing

Only grab the first disk when configured to do so
Merge pull request #65 from dirkmueller/clear_disk_before_pv

Clear disk before pv