[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 is not needed as AttributeDefinition validateAndSet already logs; see first 3 lines of https://github.com/wildfly/wildfly-core/blob/master/controller/src/main/java/org/jboss/as/controller/AttributeDefinition.java#L434

Also, if it were needed, it needs the @LogMessage annotation and the return type becomes 'void'. This is a downside of mixing the old "Logger" and "Messages" stuff in one file; easy to use the wrong syntax for the different types of messages.