Recognise Solus as a classic Linux distribution

[ikeydoherty]

This allows snapd to recognise Solus as a classic Linux distribution so
that confinement operates correctly, and ensures it behaves correctly;
in as much as rexecution for snapd will not be supported on Solus.

Signed-off-by: Ikey Doherty ikey@solus-project.com

[zyga]

LGTM

[codecov-io]

Codecov Report

Merging #3974 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #3974      +/-   ##
==========================================
+ Coverage   75.97%   75.98%   +<.01%     
==========================================
  Files         423      423              
  Lines       36505    36505              
==========================================
+ Hits        27734    27737       +3     
+ Misses       6833     6830       -3     
  Partials     1938     1938

Impacted Files	Coverage Δ
cmd/cmd.go	83.87% <100%> (ø)	⬆️
overlord/ifacestate/helpers.go	63% <0%> (+0.66%)	⬆️
cmd/snap/cmd_aliases.go	95% <0%> (+1.66%)	⬆️

---

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 1d7d79c...17d11ff. Read the comment docs.

[mvo5]

Looks good, thank you! I think this PR highlights that we need to look at is_running_on_classic_distribution() and change it so that it looks at /etc/os-release and ID=core there so that you don't have to do this. But not a concern for this PR, just an observation.

[zyga]

@mvo5 I think we didn't want to parse /etc/os-release, if that decision has changed then by all means, let's do it.

[ikeydoherty]

@mvo5 yeah whitelisting vs blacklisting is kinda of interesting in this respect.

[zyga]

@mvo5 I merged this just to keep it around in case we have any delay in your generic PR