Support ekka without epmd

priv/emqx.schema

@@ -199,7 +199,7 @@ end}.
 ]}.
 
 %% @doc The erlang distributed protocol
-{mapping, "node.proto_dist", "vm_args.-proto_dist", [
+{mapping, "node.proto_dist", "ekka.proto_dist", [
   %{default, "inet_tcp"},
   {datatype, {enum, [inet_tcp, inet6_tcp, inet_tls]}},
   hidden