Implement Worker Certificates Renewal Endpoints

[mateoflorido]

Overview

Implement the required endpoints for refreshing certificates for Kubernetes worker nodes.

Rationale

As stated in Proposal 002. We need to provide two endpoints to allow certificates refreshes in the cluster. This pull request implements the refresh-certs/plan and the refresh-certs/run for refreshing the worker nodes' certificates.

Testing

This pull request has been tested manually. Automated testing will be added in a future pull request.

root@kw-1:~# curl --unix-socket /var/snap/k8s/common/var/lib/k8sd/state/control.socket http://localhost/1.0/k8sd/refresh-certs/plan -XPOST
{"type":"sync","status":"Success","status_code":200,"operation":"","error_code":0,"error":"","metadata":{"seed":1978322463,"certificates_signing_requests":["k8sd-1978322463-worker-kubelet-serving","k8sd-1978322463-worker-kubelet-client","k8sd-1978322463-worker-kube-proxy-client"]}}
root@kw-1:~# curl --unix-socket /var/snap/k8s/common/var/lib/k8sd/state/control.socket http://localhost/1.0/k8sd/refresh-certs/run -XPOST -H "Content-Type: application/json" --data '{"seed":1978322463, "expiration_seconds": 10}'
{"type":"error","status":"","status_code":0,"operation":"","error_code":500,"error":"CSR k8sd-1978322463-worker-kubelet-serving is not issued","metadata":null}
root@kw-1:~# curl --unix-socket /var/snap/k8s/common/var/lib/k8sd/state/control.socket http://localhost/1.0/k8sd/refresh-certs/run -XPOST -H "Content-Type: application/json" --data '{"seed":1978322463, "expiration_seconds": 10}'
{"type":"sync","status":"Success","status_code":200,"operation":"","error_code":0,"error":"","metadata":null}

[bschimke95]

Amazing work @mateoflorido

Added a couple of comments.

accidentally approved

[bschimke95]

another pass

[neoaggelos]

Did a first pass. Good work overall, but see the requested changes

[bschimke95]

LGTM, great work

[neoaggelos]

Final comments, great work @mateoflorido !

[bschimke95]

LGTM!

[neoaggelos]

Final comments about the watcher restarts. It's OK to ignore the comment to start with and just a leave a NOTE if it becomes a problem in the future, rather than having huge functions, as they come with very big cyclomatic complexity