MODULES-2269: Expose notify_update setting

MODULES-2269: Expose notify_update setting

* Expose the underlying notify_update setting of the apt::settings resource
This is because not all configuration file changes should trigger an apt::update notification
* apt::pin also shouldn't result in an apt-update call
Adding a pin configuration should apply to the next apt-get update call it shouldn't trigger one itself.
* Added documentation
* Add tests for apt::conf notify_update