ceph-disk: fix various dmcrypt bugs

Two sets of patches here: the first few just make ceph-disk work with
dmcrypt.

The second set make 'ceph-disk list' print out useful information when they
are dmcrypted volumes.  This isn't quite as clean as I would like as there
is some duplicate logic in the partiion_map code and in the list code, but
it seems to work and isn't worse than before.  In an ideal work, we would
probably have a single method that makes a full 'map' of the devices,
partitions, and file systems on them, and then print it out without mounting
things (again).  But, if the fs is already mounted, this is fast, so
whatever.

It doesn't map a dmcrypt volume if it isn't already mapped, so that could
be improved as well.
if you are just checking if there are any items in holders (e.g. it is not an empty list or not None) you can simply do:

    if holders: