* 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.
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