Redirect Chinese students to a Chinese CDN for video. BLD-1052.

Use Chinese Video CDN when users coming from China.
Issue: https://edx-wiki.atlassian.net/browse/BLD-1052

@auraz please review.
@cpennington I also would like to see key to to mapping, but default "None" is against @Lyla-Fischer intent.
I will follow up your comments here as @olmar is on vacation till Tue.