Prevent kubelet from starting before the elementalhost has been bootstrapped

[anmazzotti]

As reported by @juadk , the Kubeadm-ready ISO has kubelet starting before the host has been bootstrapped, this results in the following error:

Jul 01 09:20:30 node-001 systemd[1]: kubelet.service: Scheduled restart job, restart counter is at 71.
Jul 01 09:20:30 node-001 systemd[1]: Stopped kubelet: The Kubernetes Node Agent.
Jul 01 09:20:30 node-001 systemd[1]: Started kubelet: The Kubernetes Node Agent.
Jul 01 09:20:30 node-001 kubelet[2251]: E0701 09:20:30.106145    2251 run.go:74] "command failed" err="failed to load kubelet config file, path: /var/lib/kubelet/config.yaml, error: failed to load Kubelet config file /var/lib/kubelet/config.yaml, error failed to read kubelet config file \"/var/lib/kubelet/config.yaml\", error: open /var/lib/kubelet/config.yaml: no such file or directory"
Jul 01 09:20:30 node-001 systemd[1]: kubelet.service: Main process exited, code=exited, status=1/FAILURE
Jul 01 09:20:30 node-001 systemd[1]: kubelet.service: Failed with result 'exit-code'.

Theoretically this can be ignored, kubeadm will start the service again during the pre-flight checks, but it's nice to suppress it.

Having a condition on the custom systemd service file to check on /var/lib/kubelet/config.yaml existance should be sufficient.