add a dashboard to show PV capacity

[survivant]

to help the support team with the monitoring it should be important to include a visual aid that show the disk capacity.

I found a nice Grafana dashboard here : https://github.com/sstarcher/grafana-dashboards/blob/master/prometheus/pv.json

it show all the PVC with the used space and free space.

[survivant]

that PR should allow the user to extend the OpenEBS monitoring stack : #95

but it still could be useful to have a official dashboard from OpenEBS team that will show the PV capacity