Multi-AZ controlplane with `K0sControlPlane`

[jnummelin]

Currently it is NOT possible to create multi-AZ (failure domain) controlplanes using K0sControlPlane.

The challenge is that upstream CAPI docs are bit hazy on this area. They do NOT mention anything about failure domains on control plane contract docs.

Connecting the dots between various objects and how KubeadmControlPlane implementation uses failure domains, I decipher how things should work:

• InfraCluster provider should populate InfraCluster.status.failureDomains which CAPI core will "bubble-up" to Cluster.status.
• K0sControlPlane ctrl should check if Cluster.status.failureDomains is present
• If Cluster.status.failureDomains is present, our controller should figure out the failuredomain to use when creating new machine
  • For a new machine, select the domain with the least amount of machines; This ensures the best spread over domains
  • Set the selected failureDomain to the to-be created InfraMachine object
• If no Cluster.status.failureDomains, then create the machines without any failureDomain