purge-cluster: wipe disk using dd

purge-cluster: wipe disk using dd

`bluestore_purge_osd_non_container` scenario is failing because it
keeps old osd_uuid information on devices and cause the `ceph-disk activate`
to fail when trying to redeploy a new cluster after a purge.

typical error seen :

```
2017-12-13 14:29:48.021288 7f6620651d00 -1
bluestore(/var/lib/ceph/tmp/mnt.2_3gh6/block) _check_or_set_bdev_label
bdev /var/lib/ceph/tmp/mnt.2_3gh6/block fsid
770080e2-20db-450f-bc17-81b55f167982 does not match our fsid
f33efff0-2f07-4203-ad8d-8a0844d6bda0
```

Signed-off-by: Guillaume Abrioux <gabrioux@redhat.com>
(cherry picked from commit eeedefdf0207f04e67af490e03d895324ab609a1)
Signed-off-by: Sébastien Han <seb@redhat.com>