Allow pre-workflow hook failures to stop workflows executing

[coderigo]

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

Describe the user story

A clear and concise description of what workflow is meant to be improved.
Pre-workflow hook scripts we run perform some custom inspection of PR state using the Gitlab API and we want to optionally halt atlantis locking and workflows kicking off if a pre-workflow hook returns a non-zero exit code.

Describe the solution you'd like

A clear and concise description of what you want to happen. Consider that atlantis is used
by many people, and your particular use case might not make sense to implement in the core.

Simply allow this:

repos:
  - id: xxx
    pre_workflow_hooks:
      - run: /some/script.sh 
        must_succeed: true # defaults to false if omitted

This sort of thing we hope prevents atlantis locking pull requests that we know ahead of time aren't fit for atlantis's attention.

Describe the drawbacks of your solution

must_succeed makes less sense in a post_workflow hook so in the code this distinction must be made clear.

Describe alternatives you've considered

Considered a workflow generator script, but adds a lot of complexity and not as flexible as addint a must_succeed arg to a given pre-workflow hook.