Tolerate passing controller *objects* into Mininet()

In spite of the documentation which clearly says that `Mininet()` takes a controller *constructor*, people frequently want to pass in a controller *object*. Although explaining `partial()` might make the world a better place, for now we can just let them do this. In fact, we'll let them even pass in a list of prefabricated `Controller` objects if they so desire.

We still won't let them pass in an object which isn't a descendant of `Controller`, however.