Make the k8s store properly patch annotations

[sandhose]

Hi,

While trying out Stolon on Kubernetes, I ran into a weird issue where Stolon was unable to change the annotations on the sentinel and keeper pods. The api-server said it was trying to change some fields that were immutable. This was due to Stolon trying to replace the whole pod object instead of patching it.

This PRs changes the k8s store to properly use jsonpatch to set the annotations. This is heavily inspired by the way kubectl annotate does patch the annotations: https://github.com/kubernetes/kubectl/blob/b909fcb4a071a1a9669a9fe1f48482c848823124/pkg/cmd/annotate/annotate.go#L277-L311

[dineshba]

Code looks good to me. @sandhose How to test this feature?

[sgotti]

@sandhose Thanks for the PR. Issue #678 is probably related. I'll review it in the next days.

[ccmehk]

This look good, i want a quick patch on this

[sgotti]

@sandhose let me know when you're ready for another review (I saw you pushed an updated but it didn't fixed my comments).

If you're interested this is what I had in mind to simplify this PR: sgotti@de8eddd

Please also update the commit message to follow our conventions and better describe what this patch does:

k8s store: patch pod annotations

Use k8s client mpatch method to update pod annotations (in some versions
replacing the whole pod object will return an error)

[Josua-SR]

@sandhose I came across a similar issue when I was trying to deploy stolon, and this PR definitely solved it on my end - thank you very much!

(tried on v0.15.0)

[sgotti]

@sandhose Since there was no activity and I'd like to merge this soon I created a new PR #751 with the proposed changes.

[sandhose]

@sgotti Thanks for picking that up, I really did not had time to finish this those past weeks

[sgotti]

@sandhose Thanks to you! I left you as the author of the commit in the new PR.