buildbot: add janitor that removes older build logs

roles/buildbot/files/master.cfg

@@ -24,6 +24,7 @@
 # https://docs.buildbot.net/3.6.0/manual/configuration/builders.html
 #
 
+from datetime import timedelta
 from config import config
 
 from buildbot.plugins import *
@@ -39,6 +40,12 @@ c['db'] = {'db_url': config['dbURL']}
 c['protocols'] = {'pb': {'port': config['pbPort']}}
 c['workers'] = [worker.Worker(w[0], w[1]) for w in config['workers']]
 
+c['configurators'] = [util.JanitorConfigurator(
+    logHorizon=timedelta(weeks=4),
+    hour=6,
+    minute=0,
+)]
+
 c['www'] = dict(port=config['wwwPort'],
                 plugins=dict(waterfall_view={}, console_view={}, grid_view={}),
                 change_hook_dialects={