Evaluate Flatcar Container Linux for HETZNER Cloud

We want to evaluate and possibly integrate Flatcar Container Linux as a valid OS image for AKE.

Here's a tutorial that describes how to achieve this with Terraform: https://kinvolk.io/docs/flatcar-container-linux/latest/installing/cloud/hetzner/

A few remarks:

• Flatcar is supported by Kubespray which is also Ansible-based; it's possible to take a few of their learnings to correctly setup the OS for k3s
• it's possible that a few additional steps are needed to get Longhorn working on Flatcar