Use our mirror of Helm stable repo

What does this MR do?

Mitigate impact of the stable repo at https://kubernetes-charts.storage.googleapis.com/ disappearing after 2020-11-13 (confirmed in https://github.com/helm/charts/issues/23742#issuecomment-702128318).

To achieve this we copied the packages from official Helm repo into https://gitlab.com/gitlab-org/cluster-integration/helm-stable-archive, and used GitLab pages to serve the copied packages. (A helm repo is nothing than more than a index.yaml with urls to where the tgz packages are served)

Related issue: #220141 (closed)

Tasks

• Set up new project to archive selected Helm charts (live coding session)
  • Nginx ingress
  • Prometheus
  • Fluentd
  • Postgres
• Set up Helm repo using GitLab pages
• Add digest checks - gitlab-org/cluster-integration/helm-stable-archive!1 (merged)
• Point to this new Helm repo (this MR)

Testing ✅

On my GDK:

1. Create a new Kubernetes cluster (I used GKE)
2. Add this cluster using instructions
3. Go to the Applications tab and install Ingress, Fluentd and Prometheus.

See also for logs from an earlier testing run:

• #220141 (comment 427859946)
• #220141 (comment 427862526)
• #220141 (comment 427868626)

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.