Enable heartbeat on websocket streaming

Currently the websocket streaming aborts after some time of inactivity
of build logs. Websockets have some heartbeat mechanism we can use to
keep the connection alive.

Change-Id: I5d80fe42f0ebec96ce9802acfb9c7f09f70c20ac

zuul/web/__init__.py

@@ -83,6 +83,7 @@ class LogStreamHandler(WebSocket):
     log = logging.getLogger("zuul.web")
 
     def __init__(self, *args, **kw):
+        kw['heartbeat_freq'] = 20
         super(LogStreamHandler, self).__init__(*args, **kw)
         self.streamer = None