ceph-disk: implement 'list'

This is based on Sandon's initial patch, but much-modified.

Mounts ceph data volumes temporarily to see what is inside.  Attempts to
associated journals with osds.

Resolves: #3120
Signed-off-by: Sage Weil <sage@inktank.com>
Reviewed-by: Dan Mick <dan.mick@inktank.com>