Code review comment for ~aym-frikha/charm-mongodb:branch_1946810

I note that not every reference to default_mongodb_* is prefixed with a conditional for is_percona_source(). Is that deliberate because it's not needed in some cases, or is it maybe easier to have is_percona_source used in some initial variable setup outside of the remaining functions, so that we have the `# Global variables ` section set up with generic names for paths that are adjusted according to the package source?

Regards the CI failures, I'm running a test to see if it's due to Xenial and various Python package changes that have recently meant for some other charms that we've had to remove Xenial support.