Add COMMENT_ARGS to Pre/Post-Workflow Hook Execution Environment

[adam-verigin]

Summary

This adds the COMMENT_ARGS environment variable to pre/post workflow hook execution environments, using the same escaping as for regular workflow steps.

Use Case

At my workplace, we normally unlock projects after developers run atlantis plan since only operators can run applies in most environments (for compliance reasons) and this helps alleviate lock-contention issues. We do this using a post-workflow hook.

However, there are some scenarios where we want to allow developers to conditionally keep their plans. My idea was to use atlantis plan -- --keep-locks and handle the logic in the post-workflow hook; however, that doesn't work because post-workflow hooks don't get COMMENT_ARGS. I also tried moving the unlocking step to a workflow step, but that fails to delete the plan files:

unable to obtain working dir lock when trying to delete old plans: The default workspace at path foo/bar is currently locked by another command that is running for this pull request.
Wait until the previous command is complete and try again. 

So it seems I need to keep the unlocking logic in a post-workflow hook.

[adam-verigin]

Now that this has 2 approvals, is there anything blocking it from being merged? (I can' because I don't have write access).

[nitrocode]

Thanks @adam-verigin!