Ensure .complete file is created after vulnerability scans complete

[m1j09830]

🗣 Description

The mesa-gui was not properly tracking when vulnerability scans were completed. This was due to a missing .complete file that is generated after the scans have finished running.

💭 Motivation and context

This PR addresses issue #7.

🧪 Testing

1. Build the mesa-ops.ova using the mesa-packer repo (https://github.com/cisagov/mesa-packer.git)
2. Load the vm into VMware fusion
3. Run the All Checks option from the GUI after supplying the scope
4. Verify that the vulnerability scans are running on the backend as intended (sudo journalctl -fu mesa.service)
5. Verify the vulnerability scans are showing at Complete after they have successfully completed in the GUI

✅ Pre-approval checklist

• [ ] This PR has an informative and human-readable title.
• [ ] Changes are limited to a single goal - eschew scope creep!
• [ ] All future TODOs are captured in issues, which are referenced in code comments.
• [ ] 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.
• [ ] Tests have been added and/or modified to cover the changes in this PR.
• [ ] All new and existing tests pass.

✅ Pre-merge checklist

• [ ] Revert dependencies to default branches.
• [ ] Finalize version.