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: 
Actually keep it otherwise you have a transaction at play that doesn't end with a rollback or commit. 
See https://github.com/mozilla/crontabber/blob/master/crontabber/tests/base.py#L196 for an example.