Enforce usage of super() in __init__ methods of subclasses

This is a follow up the mail in mininet-discuss.

This patch enforces the usage of super(...) in all __init__ methods of subclasses in mininet (have not touched the examples).
It allows having multiple inheritance for the Node subclasses for example.

I will take a look at the other places where subclasses call back their superclass and add additional patches/submit another pull request if needed.