BB-900: Save card_image_url in the correct model when loading data

Description

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.

*Testing instructions:*
1. Checkout this branch on your devstack
2. Log into the course discovery shell with `make discovery-shell` and updated the course metadata:
```
./manage.py refresh_course_metadata
```
3. Go to http://localhost:18130/basket/add/?sku=8CF08E5 and check that the course image is being correctly displayed.
![its_there_now](https://user-images.githubusercontent.com/27893385/53113467-69b1db00-3520-11e9-9e3b-fbe8c44efd47.png)

*Reviewers:*

  • [ ] @kaizoku

  • [ ] EdX Reviewer TBD

Assignee

Unassigned

Reporter

Open Source Pull Request Bot

Contributor Name

Giovanni Cimolin da Silva

Repo

edx/course-discovery

Customer

Epic Link

None

OSCM Assignee

None

Platform Map Area (Levels 1 & 2)

Learner Experiences - Content Discovery

Platform Map Area (Levels 3 & 4)

None

Blended Hour Utilization Percentage

None

Priority

Unset
Configure