marketplace:nginx: expose feed api to cdn.

Bug 1047633 - Get Marketplace Feed API endpoints on CDN
r? @oremj @ngokevin 
Here are the URLs we need. Sorry if the docs weren't totally up-to-date:

/api/v2/fireplace/feed/apps/{0}/
/api/v2/fireplace/feed/brands/{0}/
/api/v2/fireplace/feed/collections/{0}/
/api/v2/fireplace/feed/shelves/{0}/
/api/v2/feed/get/