Authenticate to the /ssllogin endpoint with GSSAPI authentication.

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

def _gssapi_login(self):
        """
        Authenticate to the /ssllogin endpoint with GSSAPI authentication.

        :returns: deferred that when fired returns a dict from sslLogin
        """
        method = treq_kerberos.post
        auth = treq_kerberos.TreqKerberosAuth(force_preemptive=True)
        return self._request_login(method, auth=auth)