incluster config will be used when creating external shared informers.

previously the loopback configuration was used to talk to the server.
As a consequence a custom API server was unable to talk to the root API server.

federation/cmd/federation-apiserver/app/server.go

@@ -102,6 +102,7 @@ func NonBlockingRun(s *options.ServerRunOptions, stopCh <-chan struct{}) error {
 	if err := s.CloudProvider.DefaultExternalHost(s.GenericServerRunOptions); err != nil {
 		return fmt.Errorf("error setting the external host value: %v", err)
 	}
+	s.SecureServing.ForceLoopbackConfigUsage()
 
 	s.Authentication.ApplyAuthorization(s.Authorization)