Support for pre-deploy validation webhooks.

[rophy]

Summary

kubeapps should support an optional pre-deploy validation webhook, that DevOps (admin of kubeapps, charts, or kubernetes cluster) can be used to integration various workflows that suits their team. The validation webhook is triggered after user selects chart and provided values, but before actually deploys. The webhook should return with "ok" or "not ok", providing contexts on the reason as necessary, to be displayed on kubeapps UI.

Background and rationale

Some scenarios where this can be useful:

1. Verifying target namespaces has sufficient resourcequota
2. Obtaining approvals before actually deploying
3. Ensuring user has sufficient privilege before actually deploying

Description

1. A pre-deploy validation webhooks can be provided to kubeapps configurations. If provided...

2. After user selects a package, before actually deploys, user will need to click a "validate" button, which triggers the pre-deploy webhook.

3. Relevant information will be passed to the webhook, e.g. user, cluster, namespace, chart, user-provided values

4. The webhook is expected to return some status such as "ok" or "error", along with additional information in the payload.

5. The additional information is displayed in the kubeapps UI, so user knows what to do when "not ok"

6. User can only click "Deploy" after webhook returns "ok"

[absoludity]

Thanks @rophy . Yeah, your second case - obtaining approval before actually deploying - has been requested as a specific feature in the past, but it wasn't something we could justify including by default, so I really like the idea of providing a more general validation webhook framework so people can add such behaviour to suit their use-case.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.