Return a key to use for the connection cache

def _conn_key(self, instance, db_key, db_name=None):
        ''' Return a key to use for the connection cache
        '''
        dsn, host, username, password, database, driver = self._get_access_info(instance, db_key, db_name)
        return '{}:{}:{}:{}:{}:{}'.format(dsn, host, username, password, database, driver)