fix: respect root ca validity config option

[gruyaume]

Description

Respect root ca validity config option. We also remove the "default" value in the generate_certificate helpers to make sure the charm always provides a value.

Fixes #213

Checklist:

• [ ] My code follows the style guidelines of this project
• [ ] I have performed a self-review of my own code
• [ ] I have made corresponding changes to the documentation
• [ ] I have added tests that validate the behaviour of the software
• [ ] I validated that new and existing unit tests pass locally with my changes
• [ ] Any dependent changes have been merged and published in downstream modules
• [ ] I have bumped the version of the library