Ignore empty plans on apply

[jrouly]

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• [ ] I'd be willing to implement this feature (contributing guide)

As a repository maintainer with hundreds of projects, I would like atlantis apply to ignore projects with no changes found during plan.

Specifically, I would like:

• (auto)plan to continue to include all projects, even those with no changes
• atlantis apply to only apply projects with planned changes
• atlantis apply -p <project> to only apply the specified project

The primary drawback of this approach is that side-effecting applies which would otherwise be run, despite having no changes visible to Terraform, would no longer be run. Perhaps this should be an opt-in feature?

Our primary alternative is to run targeted applies on the projects with changes. However, if ~10 projects have changes out of ~200 total projects, even running targeted applies can take a long time. Less time than applying all 200, but more than just writing atlantis apply.

[mwos-sl]

Dear bot, please unstale this. I've got the very same need. The only aspect I'm worried about is upgrading terraform from some very old version (like 0.11 to 1.), which requires several jumps, and applies in the middle even if plan is up-to-date, so that terraform applies some migrations to the state. But because we've got a guarantee for terraform 1., I'd be ok with doing these upgrades semi-manually or temporarily disabling this feature, while having requested feature enabled day-to-day.

[mwos-sl]

So there is detailed-exitcode both in terraform for some time now: https://developer.hashicorp.com/terraform/cli/v1.1.x/commands/plan#detailed-exitcode

and also terragrunts now propagates this correctly: https://github.com/gruntwork-io/terragrunt/releases/tag/v0.69.1

So atlantis could add extra flag, which ignores empty plans by checking exit code from plan. If it is equal 0 - just ignore/remove plan. And do not post long comment in the PR. Single sentnce like (Plan up to date) would be fine though. Right now when the plan is clean, we're still getting the default atlantis message with all the instructions about plan/apply...