Dashboard setup for monitoring openebs via helm

[rajaSahil]

What this PR does?

• Added dashboard directory structure for storing dashboards json for different storage engines.

  dashboards
      ├── cStor
      │   └── cstor-pool.json
      └── Jiva

• Added cStor Pool Dashboard as an example.
• Added dashboard configmap yaml to automate the loading of dashboards.

[survivant]

There something that you could validate. When using the persistence with the stack prometheus, there is a issue with AlertManager that I found : https://github.com/prometheus-community/helm-charts/issues/437

maybe you could take a look and test that we won't have the issue when we use this monitoring chart with the persistence