Initial implementation of cron role for kubeadm cert renewal

Hi @mmeyer

I had a look at your new cron role. Here is my code review:

• You are running the renewal script with the Jupyter user. I dont thik that this user has the required privileges to perform a certificate renewal. We should run this as root.
• I would suggest to rename the script to 'renew_kubeadm_certs.sh' as this fits better into the naming scheme we used so far in RWTHjupyter.
• Please try to place the script into /user/local/bin as this would be a more appropriate location rather than the home dir.
• As we are here specifically renewing Kubernetes control plane certificates, I would move the role within roles/kubernetes/renew-kubeadm-certs/
• The certificate renewal only applies to master nodes. The cronjob should therefore only deployed to these hosts.