cephadm: remove DaemonDescription.service_id()
It just doen't work out.
DaemonDescription.service_id() is impossible to implement, as there is no clear relationship between the daemon_id and the associated service. At some point we have to make this explicit and tell the daemons which service they belong to. Right now, this is an ugly hack.
I think we have to unify the OSDSpec affinity and make it generic for all daemons.