[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 aspect of the validation is fine but if this is a server setting PROXY_LIST will be silently ignored. Also, ModelOnlyWriteAttributeHandler doesn't set reload-required, so if PROXIES is updated, the reload-required bit won't happen.