This PR fixes a small bug on the discovery service where the data loader would incorrectly save `card_image_url` to the incorrect model, resulting in courses with missing course images on services that use the discovery service (e.g. ecommerce).
*Testing instructions:* Sandbox not provisioned yet.
1. Checkout this branch on your devstack
2. Log into the course discovery shell with `make discovery-shell` and updated the course metadata:
3. Go to http://localhost:18130/basket/add/?sku=8CF08E5 and check that the course image is being correctly displayed.
[ ] @kaizoku
[ ] EdX Reviewer TBD