feature request: support canonical ingress manifests

[ElvinEfendi]

ingress-nginx currently does not parse ingress manifests like following correctly:

apiVersion: extensions/v1beta1
kind: Ingress
metadata:
  name: elvin-test
  annotations:
    kubernetes.io/ingress.class: nginx
spec:
  backend:
    serviceName: http-svc
    servicePort: http
  rules:
  - host: elvin-test.example.com
  - host: something.com

For this the controller creates the server with default backend. The code responsible for this is at

ingress-nginx/internal/ingress/controller/controller.go

Line 405 in c4ba238

if rule.HTTP == nil &&

.

One possible fix would be to normalize ingresses at

ingress-nginx/internal/ingress/controller/store/store.go

Line 602 in c4ba238

if rule.HTTP == nil {

if they don't have spec.rule.HTTP. But this will probably require a change in createUpstreams because it does not return after creating an upstream based on ing.Spec.Backend if ing.Spec.Backend is not nil. It continues to iterate through ing.Spec.Rules[].HTTP.Paths to build upstreams based on ing.Spec.Rules[].HTTP.Paths[].Backend which will be nil after the normalization above. Currently it does not get hit because of

      if rule.HTTP == nil {
        continue
      }

After normalization we would also have to change catch all server detection to rely on lack of host rather than HTTP

[ElvinEfendi]

It seems like this and #3394 can be fixued together by implementing what's being suggested in #3394. cc @alexkursell