make fileshare feature use configuration builder

fixes https://github.com/Particular/NServiceBus/issues/2257
not sure that adds any real value

 * `Nservicebus` wont all keys be relevant to `Nservicebus`. doesnt add any isolation
 * `feature` wont most keys be relevant to a `feature`. not sure it adds much isolation
 * `databus/filesharepath` all of that info is already in `FileShareDataBusPath`