Don't set a product class in child products fixtures

@codeinthehole discovered a "bug" when trialling the changes in #1441:
https://github.com/tangentlabs/django-oscar/pull/1441#issuecomment-52473882

Pleasingly, the error is just a sign of the validation doing it's job.
The variant product fixtures still had a product class set for child
products.
New success messages for child products

If a child product did not define a title, the success message for the
updating the child product and the message for it's parent were
identical, as get_title() would just use the parent's title. That can be
a bit confusing, and has been addressed in this commit.
Use named variables for translation context

This makes life easier for our translators, and was suggested in
https://github.com/tangentlabs/django-oscar/pull/1441#commitcomment-7357419