config.fog_key_prefix

This allows for easy versioning of assets by key much like S3/Cloudfront's suggested best practices (over using S3's versioning feature) where the key can be prefixed with a unique key per bundle (like a git rev SHA).

Paired with relative assets (think CSS, images, sprites, this enables a slightly easier way of creating packages to reference on each deploy.

### i.e.
`distribution.cloudfront.net/version1/javascripts/application.js`
`distribution.cloudfront.net/version2/javascripts/application.js`
v.s.
`distribution.cloudfront.net/application1.js`
`distribution.cloudfront.net/application2.js`