Patching in changes form https://code.launchpad.net/~james-page/charm-helpers/lp-1475247/+merge/265125
                                                                               
The ceph disk zapping fails sometimes when issued in a single command, as described in https://github.com/ceph/ceph/commit/fdd7f8d83afa25c4e09aaedd90ab93f3b64a677b
                                                                               
This branche's purpose is to avoid a full charmhelpers sync before release.