fix: Uses [email protected] in TF module

[Gmerold]

Description

This PR fixes TELCO-1544

CONTEXT:
By default, Juju fetches charms with base matching the host OS. It means that when running Jammy, Juju won’t use latest charms, because we’ve changed the base to Noble.

Adding an optional (as per CC006) config param base will allow using Noble (base should be set to [email protected]) regardless of the host OS.

Enforcing Noble on a Juju model configuration level is not an option, because it breaks COS integration (COS charms are not available in [email protected]

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