808461 - prevent from creating a repo in rh providers

Finally, so we do not want to allow users to create custom repos in red hat providers. This was possible from the CLI, UI has no buttons (but the check is there too).

I have tried hard to implement this kind of validation in our model, but since we are creating lots of repos during import, I have decided to move this particular check into controller. Because then I'd had to re-do like 200 tests and also it does really only make sense in the API controller (repo - create) and nowhere else.