API documentation incorrectly assigns core models

The core models seem to get associated with a random service rather than with core, and therefore have the wrong URLs. As of today, it's currently listing them as part of the `onos` service.

It's possible this is a simple file ordering, and the models get associated with the first service they're encountered with. If that's the case, then ensuring we import the core models before any other service models will be a quick fix.