title: Not able to run carbon-cache with manhole enabled
body: <p>When I try to run <code>carbon-cache.py start</code> by setting </p>
  
  <pre><code>ENABLE_MANHOLE = True
  MANHOLE_INTERFACE = 127.0.0.1
  MANHOLE_PORT = 7222
  MANHOLE_USER = admin
  MANHOLE_PUBLIC_KEY = ssh-rsa AAAAB3NzaC1yc2EAAAABiwAaAIEAoxN0sv/e4eZCPpi3N3KYvyzRaBaMeS2RsOQ/cDuKv11dlNzVeiyc3RFmCv5Rjwn/lQ79y0zyHxw67qLyhQ/kDzINc4cY41ivuQXm2tPmgvexdrBv5nsfEpjs3gLZfJnyvlcVyWK/lId8WUvEWSWHTzsbtmXAF2raJMdgLTbQ8wE=
  </code></pre>
  
  <p>I get the following error</p>
  
  <pre><code>Starting carbon-cache (instance a)
  An error has occurred: b"ConchError: ('no host keys, failing', None)"
  Please look at log file for more information.
  </code></pre>
  
  <p>Log File: </p>
  
  <pre><code>25/10/2016 13:50:18 ::     'listen%s' % (self.method,))(*self.args, **self.kwargs)
  25/10/2016 13:50:18 ::   File "/opt/graphite/local/lib/python2.7/site-packages/twisted/internet/posixbase.py", line 478, in listenTCP
  25/10/2016 13:50:18 ::     p.startListening()
  25/10/2016 13:50:18 ::   File "/opt/graphite/local/lib/python2.7/site-packages/twisted/internet/tcp.py", line 1001, in startListening
  25/10/2016 13:50:18 ::     self.factory.doStart()
  25/10/2016 13:50:18 ::   File "/opt/graphite/local/lib/python2.7/site-packages/twisted/internet/protocol.py", line 74, in doStart
  25/10/2016 13:50:18 ::     self.startFactory()
  25/10/2016 13:50:18 ::   File "/opt/graphite/local/lib/python2.7/site-packages/twisted/conch/ssh/factory.py", line 41, in startFactory
  25/10/2016 13:50:18 ::     raise error.ConchError('no host keys, failing')
  25/10/2016 13:50:18 :: twisted.conch.error.ConchError: ('no host keys, failing', None)
  </code></pre>
  
  <p>Being new to twisted, i am not able to understand how to resolve it.</p>