Support KAS Ingress on an entirely separate subdomain

Summary

As discussed in https://gitlab.com/gitlab-com/gl-infra/infrastructure/-/issues/11581#note_434468216 we would like to have the ability to run kas on an entirely separate domain. Currently the ingress object for kas does not allow us to do that.

Steps to reproduce

• Deploy an instance of Gitlab chart with the values.yaml below
• Note that kas is always on the same domain as the rest of the Gitlab installation

Configuration used

A link can be seen here https://gitlab.com/gitlab-com/gl-infra/k8s-workloads/gitlab-com/-/blob/ggillies/change-kas-ingress/releases/gitlab/values/pre.yaml.gotmpl

Current behavior

kas ingress still uses pre.gitlab.com

Expected behavior

kas ingress uses kas.pre.gitlab.com.

Proposal

kas ingress uses kas.<gitlab-host>. Drop option to have <gitlab-host>/-/kubernetes-agent path

Versions

• Chart: 9a6e411e
• Platform:
  • Cloud: GKE
  • Self-hosted: (OpenShift | Minikube | Rancher RKE | ?)
• Kubernetes: (kubectl version)
  • Client: 1.16
  • Server: 1.16
• Helm: (helm version)
  • Client: 2.16.4
  • Server: 2.16.4

Relevant logs

(Please provide any relevate log snippets you have collected, using code blocks (```) to format)