Fix namespaced bug of node resource, node resource should be a
non-namespaced resource as the resource namespace.
By doing this fix, node resouce can be accessed as expected, as the issue explains.
Closes issue:[#178](https://github.com/DirectXMan12/k8s-prometheus-adapter/issues/178)
This extracts the label-to-resource conversion to a separte "naming" package for
use across different providers.
Early versions of the commit were done by antoinne85 in #79.
