Added rewrite-host-header to system model

@filler I understand that the attribute is tunable at the cluster level but I'm not sure that means we have to support it here. There are other places as well (http-logging config for example) that we haven't exposed every possible configuration threw the templates. I guess I'm saying that if we don't think that people will require a per-cluster tunable option here I'm not sure that the added complexity (both in the docs and in the attributes/templates) is worth it in this case.