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._
We'd need a place to define the features we only want to enable for users unless we enable said features manually. I could see having another ENV variable `ADMIN_FEATURES` that are only enabled for Supermarket admins, though I'm thinking I'd like to create another card for that scope. The scope of this PR is mostly to lay the groundwork for getting rollout in our stack. You could also manually enable features for admins via the Rails console.