Mention to care for implicit defaults when changing control plane parameters

[pat-s]

One can change control place parameters via control_planes_custom_config. However, it is not mentioned that implicit defaults are set (e.g. kube-controller-manager-arg: flex-volume-plugin-dir=/var/lib/kubelet/volumeplugins).

If one now sets an argument in control_planes_custom_config, the control plane becomes unhealthy.

It would be great to mention in the example kube.tf that one should first look up the already defined parameters by checking /etc/rancher/k3s/config.yaml on a control plane node.

An alternative could be to return the active values in a terraform output.