[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'm not sure about this name. It's broad, which makes it useful for lots of stuff, but also kind of vague.

I thought perhaps "host-connection" but it will interfere with tab completion and it may prove overly restrictive.

I also got a bit of negative feedback once about the non-PC nature of master/slave, so I'm a bit reluctant to make "slave" a formal term in the API. But just a tiny tiny bit.