respond with 503 when there are no endpoints

* related to:
  * https://github.com/kubernetes/ingress-nginx/issues/3070
  * https://github.com/kubernetes/ingress-nginx/issues/3335
* add a 503 test
  * test a service that starts out empty
    (a.k.a. ingress-nginx controller (re-)start)
  * test scaling up (should route traffic accordingly)
  * test scaling down to empty service
  * use custom deployments for scaling test.
* provide a fix by updating the lua table (cache) of the configured backends
  to unset the backend if there are no endpoints available.

test/e2e/lua/dynamic_certificates.go

@@ -57,7 +57,7 @@ var _ = framework.IngressNginxDescribe("Dynamic Certificate", func() {
 	})
 
 	It("picks up the certificate when we add TLS spec to existing ingress", func() {
-		ensureIngress(f, host)
+		ensureIngress(f, host, "http-svc")
 
 		ing, err := f.KubeClientSet.ExtensionsV1beta1().Ingresses(f.IngressController.Namespace).Get(host, metav1.GetOptions{})
 		Expect(err).ToNot(HaveOccurred())