[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.
Actually, we might be backporting this to EAP, there is a demand for this https://bugzilla.redhat.com/show_bug.cgi?id=954017 which will also need the modcluster counterpart https://issues.jboss.org/browse/MODCLUSTER-423. Though, there is no RFE for this yet.