[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 should subclass ReloadRequiredWriteAttributeHandler and then this line should be removed. This is a standard reload-required step, but with a bit of extra validation, so it should use the handler with the full set of logic for dealing with reload-required.