Improve support for Cortex not running on Kubernetes

[jdbaldry]

Presently this mixin depends on node-exporter, kube-state-metrics, and a (Grafana?) specific Kubernetes pod scrape config for dashboards and rules. It would be great to be able to accommodate non-k8s deployments too which would mean providing alternative queries potentially limiting the exporter dependencies to just the node-exporter.

I have some patches already that I need to put together into a PR that achieve this for some of the dashboards.