[Tracking] Flatcar support on Akamai Connected Cloud (a.k.a. Linode)

[nesv]

This issue is for tracking support for supporting Flatcar on Akamai Connected Cloud (previously known as "Linode").

• [x] Implement Ignition support — https://github.com/coreos/ignition/issues/1508
• [x] Implement Afterburn support — https://github.com/coreos/afterburn/pull/1062
• [x] Pull Ignition v2.19.0 (https://github.com/coreos/ignition/issues/1876, https://github.com/flatcar/scripts/pull/2019)
• [x] Pull Afterburn v5.5.2 (https://github.com/coreos/afterburn/issues/1076, https://github.com/flatcar/scripts/pull/2041)
• [x] OEM support on Flatcar https://github.com/flatcar/scripts/pull/1806
• [x] Provide user documentation — https://github.com/flatcar/flatcar-website/pull/327
• [ ] Implement a test suite using github.com/linode/linodego
• [ ] Run test automation for the new provider before each release

[tormath1]

Ignition and Afterburn PRs are merged. Let's wait for both releases before pulling them here: https://github.com/flatcar/scripts/pull/1806

[tormath1]

Live demo has been done in the last office hours: https://www.youtube.com/live/i04yEYnDujg?feature=shared&t=295 thanks to @nesv
The current status of the support is now:

• wait for the Afterburn release (https://github.com/coreos/afterburn/issues/1076#issuecomment-2163353183)
• or go ahead with a partial support in the next release (Ignition, OEM support but no Afterburn)

[tormath1]

On the development side, everything has been merged - Akamai official images should be available in the next release. During tests, we noticed two issues with @nesv:

• /v1/ssh-keys returns nothing on the image, seems to be because it's a private image (to be confirmed): https://www.linode.com/community/questions/24938/v1ssh-keys-is-empty-when-deploying-a-private-image
• Ignition delivery is sometimes failing: https://github.com/coreos/ignition/pull/1893

Both of these issues do not seem to be Flatcar related at the moment.