Added (airgap) images as part of the download role to provide full coverage of a k3s installation

[pwntr]

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Even if internet access on the target nodes is available, the images were not part of the Ansible rollout itself. Since they are pre-packaged and available from k3s for all versions already, I added them. This has the benefit of making an airgap installation via Ansible very easy, too.

Detailed instructions for an airgap installation are included as well, plus I bumped the sample version up to the latest v1.20.2+k3s1.

Addresses #94 Fixes #108

Cheers, Peter