fix: generate new ca cert on config changed

[gruyaume]

Description

There is a bug where on ca common name config changed, the self-signed certificates charm would generate a new CA, store it in a secret but we would read the secret in the same event handler and it would be the old secret.

Now, we return after changing the root CA certificate and updating the secret and we observe the "secret_changed" event. This allows for reading the new ca certificate instead of reading the old one.

Fixes #132

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

[saltiyazan]

In general this looks good, I have only one concern to validate.
And the title of the PR is not the most accurate, we did generate the CA until now, this has not changed.

[saltiyazan]

Looks good, I'd still use a more accurate title :), we did generate the CA before this PR too, we didn't use it as the secret was not updated.