crontabberise drop_old_partitions; bug 1014128; attempt #2

This addresses bug 1014128. Again. (Previous attempt at rebasing was a horrible disaster. :unamused:) 

`?r` @peterbe and/or @rhelmer :revolving_hearts: 
not quite, see one of the other postrgres tests for the form.  I looks more like:

```
        return get_config_manager_for_crontabber(
            jobs='socorro.cron.jobs.bugzilla.BugzillaCronApp|1d',
            overrides={
              'crontabber.class-BugzillaCronApp.query': query,
              'crontabber.class-BugzillaCronApp.days_into_past': days_into_past,
            }
        )
```