ci-automation: add hetzner testing

[tormath1]

In this PR, we bring the "glue" between Flatcar CI and Hetzner Mantle implementation.

NOTE: For this specific cloud provider there is no need to implement garbage collection as the provider is providing us a temporary project to run our tests, the project is deleted right after

Testing done

CI: http://jenkins.infra.kinvolk.io:8080/job/container/job/packages/7168/cldsv/ - works on amd64 and arm64.

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• No changelog entry as it's not user facing.
• To be merged with: https://github.com/flatcar/mantle/pull/533