Fix return type in controllers.

Because of this patch:
  https://review.openstack.org/#/c/83622/1
in swift3 middleware, the return value of s3multi
should now always be an iterable (and never a Response object).

This patch fix this issue + make some refactoring in
controllers, so now:
* All the controller methods called in handle_response
  return a callable object.
* handle_response function always returns an iterable.
(typically by calling response(env, start_response).