Fix name of addon config key

[RealOrangeOne]

See https://devcenter.heroku.com/articles/platform-api-reference#add-on-info-by-app, which shows the key is config_vars, not config.

This can be bodged in using:

Addon._strs.append("config_vars")

Which shows the correct value

[codecov-commenter]

Codecov Report

Patch coverage: 35.71% and project coverage change: -0.21% ⚠️

Comparison is base (1370af5) 50.03% compared to head (5b59d33) 49.82%.
Report is 2 commits behind head on master.

❗ Your organization is not using the GitHub App Integration. As a result you may experience degraded service beginning May 15th. Please install the Github App Integration for your organization. Read more.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #132      +/-   ##
==========================================
- Coverage   50.03%   49.82%   -0.21%     
==========================================
  Files          32       32              
  Lines        1437     1449      +12     
  Branches      178      178              
==========================================
+ Hits          719      722       +3     
- Misses        703      712       +9     
  Partials       15       15              

Files Changed	Coverage Δ
heroku3/models/app.py	31.87% <30.76%> (-0.25%)	⬇️
heroku3/models/addon.py	50.00% <100.00%> (ø)

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.