Prefer unit file drop-ins to editing the unit file provided by the package manager

[jsf9k]

🗣 Description

This pull request modifies the role to use drop-ins to alter the behavior of the systemd unit file, as opposed to editing the unit file provided by the package manager.

💭 Motivation and context

This pull request resolves #80. Using a unit file drop-in is the preferred method for making alterations to units.

🧪 Testing

All automated tests pass.

✅ Pre-approval checklist

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