Syntax check for changes in grafana dashboards

[geekodour]

Grafana dashboard configmap automation was added in : #335

Since we're going to work with improving the current Grafana dashboard for prombench and test-infra in general, it'll be good to have a syntax check to avoid problems like this. also will be nice to have some kind of basic formatting tool to do the formatting aswell, something like go fmt but to format the json inside the dashboard configmaps. But syntax check is higher priority here than the formatter.

It'll also be nice to have some kind documentation on how to add new changes to prombench dashboard.

cc: @rajdas98 @drumilpatel2000