ceph-volume: add db and wal support to raw mode

ceph-volume: add db and wal support to raw mode

Using the raw mode, the OSD preparation can now be called with extra
arguments to specify a block device for either rocksdb db or wal.

The code expects the device to be available and no validation will be
performed. Also, the entire block device will be consumed.

The goal is run this on Kubernetes with Rook and OSD are running on PVC.
Users will request PVC from a certain size, the request will be
acknowledged and a block will be created, then attached to the machine.
Later on consumed by Rook's preparation job.

Signed-off-by: Sébastien Han <seb@redhat.com>