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._
Alright I think I've come up with a solution in a349107. Basically only deactivating features that have been removed from `ENV['FEATURES']` and only activating features that aren't already activated.