[v0.16.0] Switch CoreOS to Flatcar

[dominicgunn]

Changes

• Switches to use Flatcar in the v0.16.x branch, shamelessly stolen from @jorge07, thanks!

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by:
To complete the pull request process, please assign dominicgunn
You can assign the PR to them by writing /assign @dominicgunn in a comment when ready.

The full list of commands accepted by this bot can be found here.

Needs approval from an approver in each of these files:

• OWNERS

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment

[jorge07]

pre-stolen from @davidmccormick 😂

[codecov-commenter]

Codecov Report

Merging #1863 into v0.16.x will decrease coverage by 0.00%.
The diff coverage is 0.00%.

@@             Coverage Diff             @@
##           v0.16.x    #1863      +/-   ##
===========================================
- Coverage    24.42%   24.41%   -0.01%     
===========================================
  Files           98       98              
  Lines         5114     5116       +2     
===========================================
  Hits          1249     1249              
- Misses        3725     3727       +2     
  Partials       140      140              

Impacted Files	Coverage Δ
flatcar/amiregistry/amiregistry.go	0.00% <0.00%> (ø)
flatcar/amiregistry/reliable_http.go	86.36% <ø> (ø)
pkg/model/compiler.go	55.69% <ø> (ø)
pkg/model/node_pool_compile.go	60.25% <ø> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 69ed6a7...021d72f. Read the comment docs.