`ceph_disks` can only be UUIDs

[ykazakov]

The documentation says that the values of data and db for ceph_disks is a path, which is only recommended to be under /dev/disk/by-id/.
However, apperently it is expected to be a disk UUID.
This makes it impossible use devices that do not have UUID, such as /dev/bcache0.

[stgraber]

Ah yeah, we probably should just take a full path and recommend by-id be used to avoid the path varying across reboots and causing corruption.