Make `backend/settings.py` adaptable by users/administrators

Currently, the `backend/settings.py` file is not supposed to be adapted by any user/administrator. We already have a mechanism in place to replace some configurations of this file, but it's not being used for the `backend/settings.py` file so far (but for our openattic-modules-(nagios|icinga)).

This implementation shall extend the usability of `/etc/default/openattic` and `/etc/sysconfig/openattic` to also be used to replace or add configuration variables of `backend/settings.py`.