text: Title: HPA metrics client assumes all resources are namespaced
  Body: /kind bug
  
  The HPA controller's metrics client currently assumes all objects referenced in the `Object` metrics type are namespaced (unless it's namespaces themselves).  This is wrong -- we should be using a RESTMapper to figure out the scope.
  
  When we do this, we need to make certain that we scope down the HPA's default permissions so that a normal user can't use the HPA to discover metrics that they otherwise shouldn't be able to (e.g. node metrics).