ceph-disk-prepare: add initial support for dm-crypt

Keep keys in /etc/ceph/dmcrypt-keys.

Identify partition instances by the partition UUID.  Identify encrypted
partitions by a parallel set of type UUIDs.

Signed-off-by: Alexandre Marangone <alexandre.maragone@inktank.com>
Signed-off-by: Sage Weil <sage@inktank.com>
upstart: ceph-osd-activate-dmcrypt.conf

ceph-disk-activate trigger for dm-crypt volumes.

Signed-off-by: Sage Weil <sage@inktank.com>