useRole ignored on local, because routers unaware of roles locally

FOR DISCUSSION, see discussion in related topic: https://github.com/akka/akka/issues/15042

In general the question is

1) if we want to make cluster routers take an actor system / cluster or not. There's also a broader question, as this issue is somewhat related to https://github.com/akka/akka/issues/13802 (`nr-of-instances` being confusing), which may point to that cluster routers should become more specialised?

2) The issue itself can be "solved" easily by returning `0` from the `nrOfInstances`, but that makes it possible to deadLetter some initial messages sent to the router if normally (when using `allowLocal`), that would not be the case. This safety though is a bit accidental, and maybe that's fine for fixing this issue?

If we don't want to break APIs, this could still be solved, but by setting the actor system "later" from "the inside", but's going to endup pretty ugly I think...

May eventually resolve https://github.com/akka/akka/issues/15042
thanks :)