Add support to limit applied policies in automation by specifying a selector

[Nitive]

Add support for policy selector in ImageUpdateAutomation response

Selector can be specified the same way as Deployment.spec.selector (See kubectl explain Deployment.spec.selector for details)

Example:

apiVersion: image.fluxcd.io/v1alpha1
kind: ImageUpdateAutomation
metadata:
  name: update-teleport
spec:
  policySelector:
    matchLabels:
      app.kubernetes.io/component: teleport
      app.kubernetes.io/instance: teleport
  ...

This allowes to have more flexible image updates, for example, we can batch updates of different components to separate branches. This was previously discussed in #499 and fluxcd/flux2#107

[verdel]

@stefanprodan, if you have the time, I would ask you to review this PR.

[stefanprodan]

We discussed this proposal at the Flux dev meeting, we decided to include this in the upcoming Flux minor release scheduled for end of April. @darkowlzz will ask you at some point to rebase this on a diffrent branch. Thanks for your patience.

[Nitive]

@stefanprodan thanks!

[darkowlzz]

Hey @Nitive , as mentioned above, the refactored image-automation-controller code is at #647 . It's not fully ready yet but the code where you can add the policy selector and tests associated with it are pretty much ready as of today. You should be able to modify the getPolicies() function to accept the selector and use it in the list call. And there's Test_getPolicies() which you can modify and add tests for it. Once done, you can create a PR against the refactor branch.
In case you're busy at the moment, please let me know. I can adapt the code in this PR to the new branch myself.
We are hoping to release this in April. So please let us know if you'd be able to work on it before then.

[Nitive]

Hey @darkowlzz, I will do it tomorrow

[Nitive]

@darkowlzz, it's done. Can you approve workflows please so tests can run?

[darkowlzz]

Thanks for updating the code.
Can you also update the PR description as per the current implementation?

In addition to the implementation, can you also update the spec docs for v1beta2 API and document this new field in https://github.com/fluxcd/image-automation-controller/blob/refactor/docs/spec/v1beta2/imageupdateautomations.md#writing-an-imageupdateautomation-spec ?

[Nitive]

@darkowlzz, thanks for the detailed feedback! I've applied the changes you suggested

[Nitive]

I'll update the documentation shortly (update: done)

[darkowlzz]

The code changes and tests look good to me. Thanks.

Left some minor suggestions. Once they are addressed, I think we are good to merge this.

[Nitive]

@darkowlzz, fixed

[darkowlzz]

Did some manual testing and it just works.
LGTM!
Thanks.