[v0.15.0] Switch CoreOS to Flatcar

[flah00]

Changes

• use Flatcar in the v0.15.x branch, shamelessly stolen from @jorge07 and @dominicgunn, thanks!

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[jorge07]

You'll need to modify the unit for disabled auto updates. I've it in my 0.12 branch but need to prepare a PR

[flah00]

@jorge07 I am not familiar with the unit you're talking about. Can you fill me in? :)

[jorge07]

Sure. Here the original pr #1241 and here the reference for the new OS https://docs.flatcar-linux.org/os/update-strategies/#disable-automatic-updates-daemon

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[jorge07]

Great work!

[flah00]

Thanks for your help!

[dominicgunn]

Super sorry we missed this, thanks for contributing.

[dominicgunn]

/lgtm