Fix --disable-catch-all

[alexkursell]

What this PR does / why we need it:
The --disable-catch-all flag that was recently added in #3586 is supposed to disallow ingresses that create a catch all server. Normally, a catch all server is created only if an ingress both defines a default backend and has no rules, however the flag disallows any ingress that just defines a default backend. The result is that ingresses like this are blocked by the flag:

spec:
  backend:
    serviceName: echo-service
    servicePort: 8080
  rules:
  - host: example.com
    http:
      paths:
        - path: /testpath
          backend:
            serviceName: other-service
            servicePort: 5678

This PR tightens the circumstances under which the flag causes an ingress to be ignored from spec.Backend != nil to spec.Backend != nil && len(spec.Rules) == 0, as well as adding a test for the new behaviour.

cc: @ElvinEfendi

[ElvinEfendi]

What about implementing the whole logic here - including disableCatchAll flag check here.

[wayt]

agreed but I'd change the function name to something like IsAllowedIngress. wdyt?

[wayt]

Are we sure there is no case where len(spec.Rules) > 0 and it's still a catch-all Ingress?

[alexkursell]

That condition is checked at https://github.com/kubernetes/ingress-nginx/blob/master/internal/ingress/controller/controller.go#L952

[ElvinEfendi]

LGTM, please squash your commits into one before we merge this.

[ElvinEfendi]

/lgtm
/approve

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