Add fallthrough support for Kubernetes (#626)

* Add fallthrough support for Kubernetes

This enables registering other services in the same zone as
Kubernetes services. This also re-orders the middleware chain
so that Kubernetes comes before other types, in order to make
this work out-of-the-box.

* Remove extra line

middleware/kubernetes/kubernetes.go

@@ -43,6 +43,7 @@ type Kubernetes struct {
 	Selector      *labels.Selector
 	PodMode       string
 	ReverseCidrs  []net.IPNet
+	Fallthrough   bool
 }
 
 const (