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: 
Totally trivial, but you could also get a ```bool``` back if you want:
```
SELECT EXISTS(SELECT 1 FROM information_schema.tables WHERE table_name=%s);
result = cur.fetchone()[0]
```
Then ```result``` will be ```True``` or ```False```