Migrate to flatcar linux from coreos

[davidmccormick]

Changes necessary to run flatcar linux instances instead of coreos.
This is intended as a shorter term fix to address supportability with the end-of-life of CoreOS.
Longer term I would like to move to a distribution with good support for both amd64 and arm architectures, such as Ubuntu or another.

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[jorge07]

Nice one David. You were faster than me. I was working on this this afternoon. 😂

[codecov-io]

Codecov Report

Merging #1836 into v0.15.x will decrease coverage by <.01%.
The diff coverage is 0%.

@@             Coverage Diff             @@
##           v0.15.x    #1836      +/-   ##
===========================================
- Coverage    24.36%   24.35%   -0.01%     
===========================================
  Files           98       98              
  Lines         5126     5128       +2     
===========================================
  Hits          1249     1249              
- Misses        3737     3739       +2     
  Partials       140      140

Impacted Files	Coverage Δ
pkg/model/node_pool_compile.go	60.25% <ø> (ø)	⬆️
pkg/model/compiler.go	55.69% <ø> (ø)	⬆️
flatcar/amiregistry/reliable_http.go	86.36% <ø> (ø)
flatcar/amiregistry/amiregistry.go	0% <0%> (ø)

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[Fsero]

Great job @davidmccormick thanks for putting work into this 😃