code: def agent_auth(transport, username):
      
      
      agent = ssh.Agent()
      agent_keys = agent.get_keys()
      if len(agent_keys) == 0:
          return
          
      for key in agent_keys:
          print 'Trying ssh-agent key %s' % hexlify(key.get_fingerprint()),
          try:
              transport.auth_publickey(username, key)
              print '... success!'
              return
          except ssh.SSHException:
              print '... nope.'
docs: Attempt to authenticate to the given transport using any of the private
      keys available from an SSH agent.