Support for stable API versions

[varshavaradarajan]

Issue Type

• Feature enhancement

Summary

We have been using the APIs that were available in beta with K8s 1.8:

• https://github.com/kubernetes/charts/blob/master/stable/gocd/templates/ingress.yaml
• https://github.com/kubernetes/charts/blob/master/stable/gocd/templates/gocd-server-deployment.yaml etc

With k8s 1.9, some of these are out of beta. We ought to move to support K8s 1.9 onwards.