Allow applying 0 projects when disable-apply-all is used

[rickard-von-essen]

This PR introduces a new flag --empty/-e to the apply command. This makes it possible to "apply" all changes when the --disable-apply-all flag is set. Without this this you can't get such PR merged if Github has atlantis/apply as a required status check.

TODO

• Docs

Alternative solution

Instead of adding the -e flag to the apply command the check in command_runner.go#L217 can be postponed and allow atlantis apply without flags if and only if there is zero projects to apply even if the --disable-apply-all flag was used.

Fixes: #1359

[codecov]

Codecov Report

Merging #1361 (9484946) into master (1137a82) will decrease coverage by 0.08%.
The diff coverage is 38.46%.

@@            Coverage Diff             @@
##           master    #1361      +/-   ##
==========================================
- Coverage   69.99%   69.90%   -0.09%     
==========================================
  Files          74       74              
  Lines        5575     5586      +11     
==========================================
+ Hits         3902     3905       +3     
- Misses       1310     1316       +6     
- Partials      363      365       +2     

Impacted Files	Coverage Δ
server/events/command_runner.go	50.00% <16.66%> (-0.88%)	⬇️
server/events/comment_parser.go	92.02% <50.00%> (-1.96%)	⬇️
server/events/event_parser.go	84.97% <100.00%> (+0.03%)	⬆️

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[nishkrishnan]

👋 @ rickard-von-essen mind clarifying why atlantis/apply would be set as a required status check if disable-apply-all set on Atlantis?

Based on the description this seems a little bit hacky/counterintuitive but maybe I'm not seeing the usecase where this is necessary.

[rickard-von-essen]

@nishkrishnan we have a repo containing all projects for all environments. We have this setup with auto plan, disable apply all, and auto merge. Both Atlantis plan and apply are required status checks.

This works well allowing users to propagate changes through envonments at their selected pace. At the same time it ensures all changes have been applied before the PR gets automatically merging (releasing all locks allowing other changes to come through).

[nishkrishnan]

Solved by: #1323