(MODULES-10971) - Ensure `apt::keyserver` is considered when creating a default apt...

(MODULES-10971) - Ensure `apt::keyserver` is considered when creating a default apt:source

As it stands the source_key_defaults point towards the default `keyserver` value set within the params. This change makes it so that it will instead use the `keyserver` value set in the init, which will in turn default to the params value in none has been given.