[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.
Ah, so it doesn't - I have added the reload-required logic.

```
[domain@localhost:9990 /] /profile=full-ha/subsystem=modcluster/mod-cluster-config=configuration:write-attribute(name=proxy-list,value="127.0.0.1:1234")
{
    "outcome" => "success",
    "response-headers" => {
        "operation-requires-reload" => true,
        "process-state" => "reload-required"
    },
    "result" => undefined,
    "server-groups" => {"main-server-group" => {"host" => {"master" => {
        "server-one" => {"response" => {
            "outcome" => "success",
            "response-headers" => {
                "operation-requires-reload" => true,
                "process-state" => "reload-required"
            }
        }},
        "server-two" => {"response" => {
            "outcome" => "success",
            "response-headers" => {
                "operation-requires-reload" => true,
                "process-state" => "reload-required"
            }
        }}
    }}}}
}
[domain@localhost:9990 /] :reload-servers
{
    "outcome" => "success",
    "result" => undefined,
    "server-groups" => undefined,
    "response-headers" => {"process-state" => "reload-required"}
}
```