Epinio as Manager of Helm Charts, for Customization

[manno]

We want to be able to customize apps deployed via Epinio.
We previously tried this, but didn't use Helm directly, but via Tekton, which turned out to be more complicated, as Epinio was now a manager of Tekton pipelines, that hopefully resulted in installed Helm releases.

• Allow deployment template to be customized #361
• Deploy app via helm #851
• https://github.com/manno/epinio-helm-app

Another problem was, that Helm is intended to install configurable releases, not be a template for releases themselves. For example, the app name cannot be the chart name, it's the Release.Name. While you can work around this, you end up with a template for a Helm chart.

Epinio would not create the deployment, service and ingress directly, when deploying an app. Instead Epinio would use a Helm chart. Epinio would need to know how to list, install, delete, update and version Helm charts of apps.

Advantages:

• Helm is well known
• The user (an experienced operator user) could customize the chart per app
• Helm's update logic helps with unknown customizations
• Other Rancher components use Helm as an "intermediate" format, when installing manifests. (Fleet?)