Remove whitelist in praefect config

Remove the whitelist in the praefect config. In our new scheme/architecture, praefect is simply treated by gitlab-rails as another gitaly storage. Therefore we no longer need to have a whitelist, since the purpose of it was to ease migration to praefect.

With Praefect as a separate storage, we can migrate gradually without a whitelist.

As part of this, we also need to add virtual_storage_name as a config value in the praefect config, and have praefect reject RPCs that do not match that storage name.