fix for when broker returns no messages on startup

Sometimes my RabbitMQ server doesn't return all the expected keys in it's response, possibly because the values are zero. Here is the exception I get in the log:
```
[E 140923 08:32:03 web:1407] Uncaught exception GET /monitor/broker (127.0.0.1)
    HTTPServerRequest(protocol='http', host='celery.longaccess.io', method='GET', uri='/monitor/broker', version='HTTP/1.1', remote_ip='127.0.0.1', headers={'Accept-Language': 'en-US,en;q=0.8,el;q=0.6', 'Accept-Encoding': 'gzip,deflate', 'X-Requested-With': 'XMLHttpRequest', 'Host': 'celery.longaccess.io', 'Accept': '*/*', 'User-Agent': 'Mozilla/5.0 (Windows NT 6.3; WOW64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/37.0.2062.120 Safari/537.36', 'Connection': 'upgrade', 'Referer': 'https://celery.longaccess.io/monitor', 'Cookie': 'user="2|1:0|10:1411461120|4:user|24:a2tAbG9uZ2FjY2Vzcy5jb20=|9b1f9e2d9c7fa3b13843e5305d9f92e1a5ff234a159fd5d6636f91ba39758a0d"', 'If-None-Match': '"98ea1c919cc9b14826071bda9946984bcaa66201"'})
    Traceback (most recent call last):
      File "/var/www/deepfreeze.io/env/lib/python2.7/site-packages/tornado/web.py", line 1334, in _execute
        result = yield result
      File "/var/www/deepfreeze.io/env/lib/python2.7/site-packages/tornado/gen.py", line 628, in run
        value = future.result()
      File "/var/www/deepfreeze.io/env/lib/python2.7/site-packages/tornado/concurrent.py", line 109, in result
        raise_exc_info(self._exc_info)
      File "/var/www/deepfreeze.io/env/lib/python2.7/site-packages/tornado/gen.py", line 633, in run
        yielded = self.gen.send(value)
      File "/home/kouk/flower/flower/views/monitor.py", line 101, in get
        data[queue['name']] = queue['messages']
    KeyError: 'messages'
```