Small issues with `prerun.py.override`

I just logged [ckan/ckan-docker-base#24](https://github.com/ckan/ckan-docker-base/issues/24), wherein I identified some small issues with `prerun.py`, which is used by the CKAN Docker image to set some things up. A copy of this file is included in this repository as the override template, [ckan/setup/prerun.py.override](https://github.com/ckan/ckan-docker/blob/master/ckan/setup/prerun.py.override).

Whatever changes end up happening in `ckan-docker-base` should probably ported to the override file as well.