code: def fetch_token(self):
          
          grant_type = 'client_credentials'
  
          channel = yield self._tvm.ticket_full(
              self._client_id, self._client_secret, grant_type, {})
          ticket = yield channel.rx.get()
  
          raise gen.Return(self._make_token(ticket))
docs: Gains token from secure backend service.
  
          :return: Token formatted for Cocaine protocol header.