FR-5549 - Make sure customized cloud-init files are valid

[upils]

Fixes LP: #2032585

[codecov]

Codecov Report

Merging #147 (81450d5) into main (40cf462) will decrease coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #147      +/-   ##
==========================================
- Coverage   87.33%   87.32%   -0.02%     
==========================================
  Files          11       11              
  Lines        2440     2438       -2     
==========================================
- Hits         2131     2129       -2     
  Misses        289      289              
  Partials       20       20              

Files	Coverage Δ
internal/statemachine/classic_states.go	97.86% <100.00%> (-0.01%)	⬇️

[sil2100]

By principle, I think what we want is to warn or error-out on users submitting wrong cloud-init file contents instead of implicitly fixing the input for them. I know this might be a bit of an additional burden for users, but the idea behind the cloud-init customization was always: allow providing 1-to-1 contents for the respective cloud-init metadata. So I'm thinking of these as 1:1 content per what is in the image definition. Adding things without the users knowing seems weird. Also, it might encourage people to misunderstand how proper cloud-init files should be structured - if someone forgets to add the #cloud-init header, then they might forget to do it when they're not using ubuntu-image.

So I'd recommend us erroring out, or maybe only warning, when someone provides an 'invalid' cloud-init file instead of doing it for them.

[upils]

OK, I understand the position and I agree with having a 1:1 content between a proper cloud-init and the content of the image definition.
I will fix this to error-out because I really fear a simple warning might be too easy to ignore. I suppose that if users set a cloud-init config they expect it to be valid in the end and not end up in the situation we encounter right now.

[sil2100]

Looking good! Test coverage is good as well. Let me merge this.