Make backup parameter optional

[alessfg]

Proposed changes

Add backup variable to template and upload parameters. Set to false if you don't want to keep backups of your previous NGINX config files. Closes #111.

Checklist

Before creating a PR, run through this checklist and mark each as complete.

• I have read the CONTRIBUTING document
• I have added Molecule tests that prove my fix is effective or that my feature works
• I have checked that any relevant Molecule tests pass after adding my changes
• I have updated any relevant documentation (defaults/main/*.yml, README.md and CHANGELOG.md)