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.

For `--dry-run` to work with kubectl a Kubernetes cluster's apiserver is
actually used, which is unnecessary for generating these manifests. This
approach also allows further customization, such as adding labels to the
generated manifests.