Mutable Default Arugments

[BillWSY]

https://github.com/ros/executive_smach/blob/1a44c126b3e7423ed35e02d251a5146003fd6583/smach/src/smach/container.py#L122

https://github.com/google/styleguide/blob/gh-pages/pyguide.md#2123-cons

This particular case here might not cause any issue, but still, it is a terrible programming style and a common issue for Python beginners.