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: 
Here's a stack overflow link that discusses this a bit:
http://stackoverflow.com/questions/13793399/passing-table-name-as-a-parameter-in-psycopg2

That helps for seeing if the table already exists, but there isn't a great way to substitute the table name (or other things like SQL keywords) without doing string interpolation that I know of.