Send overwrite: true in creation/update requests because else the Grafana API does weird stuff

src/grafana/dashboards.go

 func (c *Client) CreateOrUpdateDashboard(slug string, contentJSON []byte) (err error) {
 	reqBody := dbUpdateRequest{
 		Dashboard: rawJSON(contentJSON),
-		Overwrite: false,
+		Overwrite: true,
 	}
 
 	reqBodyJSON, err := json.Marshal(reqBody)