Don't use WatchedDatacenters in the xds code as thsoe get nil'ed out prior to sending to xds

agent/xds/listeners.go

@@ -323,7 +323,7 @@ func (s *Server) makeGatewayListener(name, addr string, port int, cfgSnap *proxy
 
 	// TODO (mesh-gateway) - Do we need to create clusters for all the old trust domains as well?
 	// We need 1 Filter Chain per datacenter
-	for dc := range cfgSnap.MeshGateway.WatchedDatacenters {
+	for dc := range cfgSnap.MeshGateway.GatewayGroups {
 		clusterName := DatacenterSNI(dc, cfgSnap)
 		filterName := fmt.Sprintf("%s_%s", name, dc)
 		dcTCPProxy, err := makeTCPProxyFilter(filterName, clusterName, "mesh_gateway_remote_")