Add `wal` option to `ceph_disks`

[ykazakov]

I think since data and db options can be separately specified for ceph_disks then the wal option should be supported as well with the more general version of the ceph-volume command in osd.yml:

ceph-volume lvm create --data {vg/lv} --block.wal {partition} --block.db {/path/to/device}

[stgraber]

Yeah, that'd be fine and should be pretty trivial to add