Add cherrypy to built-in logging config

Exceptions in zuul-web will be logged to cherrypy.error at ERROR
level.

Change-Id: Iac060974a3e56ffa871be897fbf75fb61032db0b

zuul/ansible/logconfig.py

@@ -110,6 +110,10 @@ _DEFAULT_SERVER_LOGGING_CONFIG = {
             'handlers': ['console'],
             'level': 'INFO',
         },
+        'cherrypy': {
+            'handlers': ['console'],
+            'level': 'WARN',
+        },
     },
     'root': {'handlers': []},
 }