fix(users): users should always be created as system users

[mdschmitt]

PR progress checklist (to be filled in by reviewers)

• [x] Changes to documentation are appropriate (or tick if not required)
• [ ] Changes to tests are appropriate (or tick if not required)
• [ ] Reviews completed

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What type of PR is this?

Primary type

• [ ] [build] Changes related to the build system
• [ ] [chore] Changes to the build process or auxiliary tools and libraries such as documentation generation
• [ ] [ci] Changes to the continuous integration configuration
• [ ] [feat] A new feature
• [x] [fix] A bug fix
• [ ] [perf] A code change that improves performance
• [ ] [refactor] A code change that neither fixes a bug nor adds a feature
• [ ] [revert] A change used to revert a previous commit
• [ ] [style] Changes that do not affect the meaning of the code (white-space, formatting, missing semi-colons, etc.)

Secondary type

• [ ] [docs] Documentation changes
• [ ] [test] Adding missing or correcting existing tests

Does this PR introduce a BREAKING CHANGE?

Yes. If users already exist and this formula tries to mandate that they be system users, I think it'll try to change their existing uid, which is bad or should fail or both.

Related issues and/or pull requests

Fixes #70

Describe the changes you're proposing

All users created by this formula are expected to be users that run exporters or prometheus itself. All should be system users, not added with regular user uids.

Pillar / config required to test the proposed changes

N/A

Debug log showing how the proposed changes work

Documentation checklist

• [ ] Updated the README (e.g. Available states).
• [ ] Updated pillar.example.

Testing checklist

• [ ] Included in Kitchen (i.e. under state_top).
• [ ] Covered by new/existing tests (e.g. InSpec, Serverspec, etc.).
• [ ] Updated the relevant test pillar.

Additional context

[saltstack-formulas-travis]

:tada: This PR is included in version 5.6.1 :tada:

The release is available on GitHub release

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