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.
Please be sure to deprecate this in the EAP 6.4 code, as that is where we really need to advertise deprecation.