Remove old stats from whisper

Over time, graphite seems to have acquired a range of keys that are no
longer updated and can be quite confusing when trying to find data.

This small cron job (cribbed from suggestions in [1]) should remove
all old data that hasn't been updated in nine months (this should be
long-enough to cover all manner of periodic jobs)

[1] http://stackoverflow.com/questions/9587161/how-to-cleanup-the-graphite-whispers-data

Change-Id: I598b9d370a337866e9086cbe0238569da70aa471
Merge "Remove old stats from whisper"