Dependencies: nginx-ingress chart

Split from #1050 (closed)

Our nginx-ingress chart fork (under charts/nginx) makes use of

• k8s.gcr.io/defaultbackend 1.4 (source not located)
• quay.io/kubernetes-ingress-controller/nginx-ingress-controller

The origin of defaultbackend has not been located, and appears to not follow SemVer.

nginx-ingress-controller from comes from kubernetes/ingress-nginx and follows SemVer. This should use type: git-repo. We should remain aware of the possible impacts to changes the controller, and newer requirements of Kubernetes or ConfigMaps.