def validate_rmq_ssl_enabled_units(self, sentry_units, port=None):
        """Check that ssl is enabled on rmq juju sentry units.

        :param sentry_units: list of all rmq sentry units
        :param port: optional ssl port override to validate
        :returns: None if successful, otherwise return error message
        """
        for sentry_unit in sentry_units:
            if not self.rmq_ssl_is_enabled_on_unit(sentry_unit, port=port):
                return ('Unexpected condition:  ssl is disabled on unit '
                        '({})'.format(sentry_unit.info['unit_name']))
        return None
Check that ssl is enabled on rmq juju sentry units.

        :param sentry_units: list of all rmq sentry units
        :param port: optional ssl port override to validate
        :returns: None if successful, otherwise return error message