title: Redis sentinel doesn't auto-discover other sentinels
body: <p>I have redis configured as master with two slaves, each on a separate box. I also have a sentinel process running on each box. It's the setup as described in the documentation here:</p>
  
  <p><a href="http://redis.io/topics/sentinel#example-2-basic-setup-with-three-boxes" rel="noreferrer">http://redis.io/topics/sentinel#example-2-basic-setup-with-three-boxes</a></p>
  
  <p>Each of the sentinels can connect to my master, and can see the slaves. They are able to independently detect if the master or slaves go down. The problem is that the sentinels can't detect each other.</p>
  
  <p>I have verified that each sentinel is publishing a message to the <code>__sentinel__:hello</code> channel as expected, but it seems that none of them are actually receiving the messages from the other ones.</p>
  
  <p>How do I get the sentinels to see each other?</p>