WFLY-2090  WFLY-2171 WFLY-2571 Make wildfly-mod_cluster-undertow module a separate subsystem

https://issues.jboss.org/browse/WFLY-2090
https://issues.jboss.org/browse/WFLY-2171
https://issues.jboss.org/browse/WFLY-2571

Makes mod_cluster work again.