Swap out purely ENV based feature flags for rollout

:fork_and_knife: Implement rollout with features currently enabled by an ENV variable so we may take advantage of some of rollouts more robust features going forward.

_This shouldn't be merged until https://github.com/opscode-cookbooks/supermarket/pull/47 is merged and released._
At that point, we may as well just use environment variables, eh? :smile:

I'm wary only because it *feels* weird to use an out-of-sync node attribute (environment variables) to dictate the value of a centralized attribute (the actual feature flag). In practice it may not be a problem! However, I think the solution for this maybe-problem is to manage feature flags outside of the app's lifecycle.

One way to do it would be by running `rails runner 'ROLLOUT.stuff'` from a node. Alternatively, we could use something like [RolloutUI](https://github.com/jrallison/rollout_ui) and constrain the mounted app to admins.