Insure only destroyable nodes are destroyed

def destroy(self):

        """Insure only destroyable nodes are destroyed"""

        node = self.node
        if not config.is_node_destroyable(node.name):
            logger.error('node %s has non-destroyable prefix' % node.name)
            return False
        logger.info('destroying node %s' % node)
        return node.destroy()