Set up log removal on buildbot master

[larsbergstrom]

The buildbot manual (http://docs.buildbot.net/0.8.1/full.html) recommends rules like:

     @weekly cd BASEDIR && find . -mindepth 2 i-path './public_html/*' -prune -o -type f -mtime +14 -exec rm {} \;
     @weekly cd BASEDIR && find twistd.log* -mtime +14 -exec rm {} \;

I couldn't quite make this work (the i-path thing to exclude public_html kept giving me errors). I also think we should do 5 days instead of 2 weeks, as we don't have that much disk space :-)

We ran out of disk again today, though luckily it was during normal working hours, so nobody had to do a late-night recovery.

CC @edunham

[larsbergstrom]

owait, there's already a PR; will comment there...