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: 
I'm guessing this has to do with helping to prevent injection attacks by leveraging protections in the DB library?  If so, is there a way to do this for `TABLE` as well (i.e. prevent it from being wrapped in single-quotes when parsed by `cur.execute`)?

Also, since the single-quotes are added, they'd need to be removed `statement` where appropriate.