Apply annotations to default location

[alexkursell]

What this PR does / why we need it:

Right now, support for ingresses like the following is broken:

apiVersion: extensions/v1beta1
kind: Ingress
metadata:
  name: example-ingress
  annotations:
    nginx.ingress.kubernetes.io/proxy-buffer-size: "8k"
spec:
  backend:
    serviceName: echo-service
    servicePort: 8080
  rules:
  - host: example.com

While it will route requests for example.com to the service, none of the annotations are applied. This is in contrast to the behaviour of

apiVersion: extensions/v1beta1
kind: Ingress
metadata:
  name: example-ingress2
  annotations:
    nginx.ingress.kubernetes.io/proxy-buffer-size: "8k"
spec:
  rules:
  - host: example.com
    http:
      paths:
        - path: /
          backend:
            serviceName: echo-service
            servicePort: 8080

where the annotations are applied. If we are going to support ingress definitions like the first one here, we should support annotations for them as well.

Which issue this PR fixes
fixes #3520

Special notes for your reviewer:

This PR reverses #1233. I don't have any context as to why that change was made, so if the reasons why are still relevant it would be good to discuss.

cc: @ElvinEfendi , @aledbf

[ElvinEfendi]

@alexkursell please write an e2e test showing the issue.

[ElvinEfendi]

test/e2e/defaultbackend/default_backend_annotations.go

I think this should rather be test/e2e/defaultbackend/with_hosts.go. It's about supporting scenario when default backend is being used for give hosts. And making annotations work properly is only one part of it.

[ElvinEfendi]

After this PR we should DRY this code path - having annotation assignment being duplicated in multiple place is error prone and hard to maintain.

[ElvinEfendi]

/lgtm
/approve

[k8s-ci-robot]

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