core: replace GetMiddleware (#885)

* core: replace GetMiddleware

See the discussion in #881. GetMiddleware would add a `nil` middleware
to the callstack thereby breaking functionality.

This PR drops it in favor of RegisterHandler which is a completely
standalone registry for middleware that want to let it self know to
other middleware.

Currenly *autopath* uses this to call *kubernetes*'s AutoPath method
for dynamic autopathing.

* Drop GetMiddleware

* Register metrics

* drop the panic

middleware/kubernetes/setup.go

@@ -51,6 +51,9 @@ func setup(c *caddy.Controller) error {
 		return kubernetes
 	})
 
+	// Also register kubernetes for use in autopath.
+	dnsserver.GetConfig(c).RegisterHandler(kubernetes)
+
 	return nil
 }