use Forwardable for all the feature flag delegation

Be clear about:

1. Cannot instantiate a Feature object (module instead of class)
2. List of methods delegated to the "feature flag adapter" which
   happens to be Rollout in our implementation.

Signed-off-by: Robb Kidd <rkidd@chef.io>

src/supermarket/app/lib/feature.rb

@@ -1,21 +1,13 @@
 require 'redis'
 
-class Feature
-  @rollout = nil
-
-  def self.active?(*args)
-    rollout.active?(*args)
-  end
+module Feature
+  extend Forwardable
 
-  def self.activate(*args)
-    rollout.activate(*args)
-  end
+  module_function(*def_delegators('Feature.adapter', :active?, :activate, :deactivate))
 
-  def self.deactivate(*args)
-    rollout.deactivate(*args)
-  end
+  @rollout = nil
 
-  def self.rollout
+  def self.adapter
     return @rollout if @rollout
 
     redis_url = ENV['REDIS_URL'] || 'redis://localhost:6379/0/supermarket'
@@ -42,6 +34,4 @@ def self.rollout
 
     @rollout
   end
-
-  private_class_method :rollout
 end