Install via `snap` on Ubuntu

[jsf9k]

🗣 Description

This pull request modifies the Ansible role to install the Amazon SSM agent via snap on Ubuntu.

💭 Motivation and context

Resolves #56.

🧪 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] All relevant repo and/or project documentation has been updated to reflect the changes in this PR.
• [x] Tests have been added and/or modified to cover the changes in this PR.
• [x] All new and existing tests pass.