Support customized kubelet configuration.

[ralimi]

Support is added for both the alpha1 and alpha3 config.yaml formats.
Before the alpha3 format, kubelet configuration could be added via
kubeadm_extra_config. However, that doesn't work with the alpha3
format since kubelet configuration is passed via a separate YAML
document with kind == KubeletConfiguration:
https://godoc.org/k8s.io/kubernetes/cmd/kubeadm/app/apis/kubeadm/v1alpha3#hdr-Kubeadm_init_configuration_types

[davejrt]

Looks good, can you please document the param in the README as well?

[ralimi]

Oops. Done.

[davejrt]

LGTM

[nickperry]

This approach only works for kubelet config that is not critical for starting the kubelet up in the first place. I think this needs to be made clear in the documentation.

For example, in our air-gapped environments we need to be able to tell the kubelet to source the infra / pause container from an internal repo rather than trying in vain to get it from the Internet. We do this with the 'pod-infra-container-image' argument to Kubelet. However the Kubelet can't get to a point where dynamic config is viable without infra pods being able to get their pause container.

Instead, of using this new kubeadm_extra_config param which renders settings under
apiVersion: kubelet.config.k8s.io/v1beta1
kind: KubeletConfiguration
section of config.yaml

we need to specify pod-infra-container-image under

apiVersion: kubeadm.k8s.io/v1alpha3
kind: InitConfiguration
nodeRegistration:
kubeletExtraArgs:

and

kind: JoinConfiguration
nodeRegistration:
kubeletExtraArgs:

in config.yaml. The module does not currently support setting kubeletExtraArgs, but I would like it to. We can't use the generic kubeadm_extra_config as it only gets merged with the yaml under kind: InitConfiguration, but not under kind: JoinConfiguration, meaning our worker nodes don't get the configuration they need.