❇️ Support airgap install

[nasusoba]

K3s is supporting air-gapped installations(see k3s Air-Gap Install). This PR allowed the user to run CAPI k3s in an air-gapped environment.

Summary

• Add a boolean config AirGapped to KThreesAgentConfig. If set to true, in the cloudinit file, it will use INSTALL_K3S_SKIP_DOWNLOAD=true INSTALL_K3S_EXEC='server' sh /opt/install.sh instead of curl -sfL https://get.k3s.io | INSTALL_K3S_VERSION=%s sh -s - server.
• Small Modification to avoid code duplication:
  • Merge controlPlaneCloudJoin and controlPlaneCloudInit
  • Introduce a variable SentinelFileCommand (the command echo success to bootstrap-success.complete)
• Add samples/air-gapped/README.md for guiding the user for airgap install setup, also I use it for validation

Note

• Currently, we are still using etcdproxy and it needs to be downloaded from Internet. But as etcdproxy is deprecating, the feature will be fully supported after etcdproxy removed