Make GET /flavors response structure consistent

Return a JSON object instead of a JSON array as the response representation.
This is consistent with the response representation of GET /services. Further,
it allows for the possibilty of adding a top-level "links" property in the
future.

Change-Id: I64fabfb5974eb928d84f937b125d4826cc86b78a

poppy/transport/pecan/controllers/v1/flavors.py

@@ -41,7 +41,9 @@ class FlavorsController(base.Controller):
         flavor_list = [
             flavor_response.Model(item, pecan.request) for item in result]
 
-        return flavor_list
+        return {
+            'flavors': flavor_list
+        }
 
     @pecan.expose('json')
     def get_one(self, flavor_id):