Authenticate to the /ssllogin endpoint with Client SSL authentication.

        :returns: deferred that when fired returns a dict from sslLogin

def _ssl_login(self):
        """
        Authenticate to the /ssllogin endpoint with Client SSL authentication.

        :returns: deferred that when fired returns a dict from sslLogin
        """
        method = treq.post
        agent = self._ssl_agent()
        return self._request_login(method, agent=agent)