nodes: check switch from debian to fedora coreos

[herzogf]

When homek8s started there were no docs mentioning k3s support for fedora coreos (https://getfedora.org/en/coreos/). This + the fact that we use raspbian on the raspberry pi + the fact that fedora coreos was not supported on raspberry pi (and we want to leave the door to hybrid arch clusters open) lead to the decision to use debian as OS on the nodes as well.

In this issue coreos/fedora-coreos-tracker#329 the author mentions that he runs k3s on fedora coreos and even has published his pxe / tftp files for fedora coreos (fcos): https://github.com/stellirin/coreos-k3s-ceph/tree/master/tftp (forked it to preserve the current state: https://github.com/herzogf/coreos-k3s-ceph)

We definitely need to evaluate switching to fedora coreos to a) lower the runtime overhead b) speed up provisioning new nodes and c) simplify OS patching.

[herzogf]

fedora coreos PXE install instructions: https://github.com/coreos/coreos-installer#installing-from-pxe

[herzogf]

fedora coreos is not yet supported on raspberry pi, see coreos/fedora-coreos-tracker#258