CLA-677 Refactor subscriptions so that subscriptions are de-registered on deletion of a node. Still getting 2 deletion notifications for some reason. Make ZooKeeper the default with the config file able to specify ZooKeeper or doozer. Skynet Service Registry now uses the ZooKeeper or Doozer registries, rather than inherits from. An empty string value now returns as '' rather than false. When a leaf node is deleted and its parents no longer have any children or a value, they are now automatically deleted. Create registry root path on startup to ensure it is present and so that registrations work against it.