[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.
Better is to have a consistent structure and create undefined nodes: 

ModelNode targetType = target.get(TYPE);
if (type != null) {
    targetType.set(type);
}
ModelNode targetAddress = target.get(ADDRESS);
if (address != null) {
    targetAddress.set(address);
}