Ceph

Calling:
`root@juju-c268c9-3:~# sudo -u ceph ceph-disk prepare --data-dir /srv/ceph --dmcrypt`
fails with a bad assertion:
`Traceback (most recent call last):
File "/usr/sbin/ceph-disk", line 9, in <module>
load_entry_point('ceph-disk==1.0.0', 'console_scripts', 'ceph-disk')()
File "/usr/lib/python2.7/dist-packages/ceph_disk/main.py", line 4995, in run
main(sys.argv[1:])
File "/usr/lib/python2.7/dist-packages/ceph_disk/main.py", line 4948, in main
main_catch(args.func, args)
File "/usr/lib/python2.7/dist-packages/ceph_disk/main.py", line 4973, in main_catch
func(args)
File "/usr/lib/python2.7/dist-packages/ceph_disk/main.py", line 1774, in main
Prepare.factory(args).prepare()
File "/usr/lib/python2.7/dist-packages/ceph_disk/main.py", line 1762, in prepare
self.prepare_locked()
File "/usr/lib/python2.7/dist-packages/ceph_disk/main.py", line 1793, in prepare_locked
self.lockbox.prepare()
File "/usr/lib/python2.7/dist-packages/ceph_disk/main.py", line 2358, in prepare
verify_not_in_use(self.args.lockbox, check_partitions=True)
File "/usr/lib/python2.7/dist-packages/ceph_disk/main.py", line 797, in verify_not_in_use
holders = is_held(dev)
File "/usr/lib/python2.7/dist-packages/ceph_disk/main.py", line 769, in is_held
base = get_dev_name(dev)
File "/usr/lib/python2.7/dist-packages/ceph_disk/main.py", line 546, in get_dev_name
assert path.startswith('/dev/')
AssertionError`

Running it without the dmcrypt flag (`sudo -u ceph ceph-disk prepare --data-dir /srv/ceph`) works fine.