[WFLY-457] Replace mod_cluster proxy-list attribute with list of outbound socket bindings

https://issues.jboss.org/browse/WFLY-457

Needs https://github.com/wildfly/wildfly/pull/6420 merged first and then rebase.
If the config is no longer supported on a current-release server, this block should be restored, but throw an OFE . Otherwise we're silently ignoring the user's intent (other than the deprecation logging.) This method only gets called on a server, so if you get here you know you are on a current-release server. 

This follows the pattern we are using for things like the legacy web, cmp and jaxr subsystems -- forgiving in Stage.MODEL to support managing legacy HCs but we fail if they attempt use on a current-release server.