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.
This attribute is a model reference to a name of an outbound socket binding resource. We don't allow expressions for model references because we need to have the ability to validate the model reference in Stage.MODEL, before expressions can be safely resolved. So please remove the setAllowExpression in both PROXY and PROXIES.