Make database open attempts configurable also via config file

[steinex]

Description of the Change

This makes the global command line flag -db-open-attempts a configuration option in aptly.conf (as databaseOpenAttempts).

We'd rather configure it there instead of adding the flag to every invocation of aptly in our CI/CD setup.

I couldn't test this change thoroughly, but I oriented on how it was done for other, similar flags/options.

Thanks!

Checklist

• unit-test added (if change is algorithm)
• functional test added/updated (if change is functional)
• man page updated (if applicable)
• bash completion updated (if applicable)
• documentation updated
• author name in AUTHORS

[smira]

lgtm, thanks! 👍

[codecov]

Codecov Report

Merging #869 into master will decrease coverage by 0.02%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master     #869      +/-   ##
==========================================
- Coverage   63.84%   63.81%   -0.03%     
==========================================
  Files          54       54              
  Lines        6651     6654       +3     
==========================================
  Hits         4246     4246              
- Misses       1881     1884       +3     
  Partials      524      524

Impacted Files	Coverage Δ
utils/config.go	64.7% <ø> (ø)	⬆️
context/context.go	11.14% <0%> (-0.14%)	⬇️
deb/remote.go	60.92% <0%> (+0.12%)	⬆️

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