[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.
@bstansberry Unless my logic is flawed, this will address the first concern in https://github.com/wildfly/wildfly/pull/6498#discussion_r15912742:

* !isNormalServer(): if both are used, the controller will throw an OFE
* if used on old slave - no problem
* isNormalServer(): if PROXY_LIST is used on new slave

I have changed the message accordingly.

Thanks!