Zap disk before running ceph-disk prepare

We remove all the partitions, label and re-create something clean prior
to prepare the design. This will help solving many issues with existing
disks or while scratching/deploy test environments often.

Signed-off-by: Sébastien Han <sebastien.han@enovance.com>
Merge pull request #158 from leseb/zap-disk-before-osd-creation

Zap disk before running ceph-disk prepare