Make build user support a single build environment

[dav3r]

🗣 Description

This PR modifies the build user that is created by this module such that the user can only build AMIs in a single environment.

💭 Motivation and context

In the past, we had a single build user that was capable of building AMIs in both a staging and production environment. In our target (near-future) state, each packer repo will have one build user per environment (e.g. dev, staging, production, etc.)

This supports our overall goal of a COOL system where each environment's IAM users exist in an AWS account specific to that environment and no others.

Note

I am marking this PR as "blocked" until the related skeleton-packer PR (cisagov/skeleton-packer#393) has been reviewed and approved, to minimize the amount of time that our cisagov packer repos are out of sync with the latest changes in this module.

🧪 Testing

I used the version of the module in this PR to successfully create dev, staging, and production build users for cisagov/skeleton-packer#393.

✅ Pre-approval checklist

• This PR has an informative and human-readable title.
• Changes are limited to a single goal - eschew scope creep!
• All relevant type-of-change labels have been added.
• I have read the CONTRIBUTING document.
• These code changes follow cisagov code standards.
• All relevant repo and/or project documentation has been updated
  to reflect the changes in this PR.
• All new and existing tests pass.

✅ Post-merge checklist

• Update all cisagov *-packer repos to reflect the changes in this PR (see Support new COOL environment structure skeleton-packer#393)
• Create/update dev/staging/prod build users for all cisagov *-packer repos

[jsf9k]

Approved, with one question.