Heptapod: generic implementaion for apply_hashed_storage_setting
It will work for any subclass that implements `gitlab_ctl` and `rake` (currently corresponding to the `@needs.services` generator), hence for `GdkHeptapod` as well as `DockerHeptapod`. This implementation is tighter in that we make sure that Redis, where the annoying cache should reside, is stopped before restarting everything. The restart of Rails dominates execution time anyway. Of course, at some point we could find a gentler way to force the previous setting out of the caches, but at least we can now run hashed storage related tests in GDK context. We don't need `restart_rails()` anymore, a more controlled style is wishable anyway.