Course teams can turn off Chinese Caching from studio (BLD-1207)

Allow turning off Chinese CDN from Studio.
**Issue**: [BLD-1207](https://openedx.atlassian.net/browse/BLD-1207)

Please review: @tymofij 
Yes, this will show in Advanced settings:

![screenshot](https://cloud.githubusercontent.com/assets/123786/3860917/15c7bb8e-1f29-11e4-9017-73ec702ef265.png)

When the switch is on videos are served from CDN servers when such a country-specific server is configured. At the moment we have only one, for China.