Datasource links were duplicated in the grafana dashboads. This now also
allows exporting grafana dashboards from the UI and just dropping them
into the assets directory and they will be wrapped by the manifest
generation script.

git-subtree-dir: contrib/kube-prometheus
git-subtree-mainline: 050ca21276696c8603375c699513ec487301ed62
git-subtree-split: 81c0d2f4