Add a Grafana Dashboard to visualize metrics exported by node-exporter

[peppepetra]

Is your feature request related to a problem? Please describe.
To use metrics exported by the node-exporter I need to be able to visualize in Grafana

Describe the solution you'd like
Include in the charm a Grafana dashboard like Node Exporter Full and ship it to Grafana through the dashboard grafana-dashboard interface

Describe alternatives you've considered
Consider if you want to use the grafana_dashboard interface to ship directly to grafana-k8s in COS instead of passing through the Cos-proxy.