[WFLY-547] track uncleanly unregistered slave hosts

https://issues.jboss.org/browse/WFLY-547

This requires some tricks with registering runtime resource, so if this and the model structure is OK, then i can add more information we expose.
I wonder if this should be down a level, in core-service=management or something. It's ok here, but it becomes a bit prominent and may prove confusing for admins who aren't really interested in this.

Just an aside: admin-only hosts can connect now without actually registering, and the connection is maintained so they can pull down previously ignored config data if they need it. I'm considering exposing those in the resource tree as well, e.g. /admin-only-host=x. This will allow the admin so configure them via the DC instead of having to directly connect. I mention this because it's another aspect of how we represent slaves in the resource tree.