Pelorus Grafana dashboard does not work if long term storage isn't used

[themoosman]

Running pelorus without long term storage breaks Grafana. This is because when long term storage isn't run, the service https://thanos-pelorus.pelorus.svc:9092 isn't created. However, the Grafana dashboard still points to that service.

[etsauer]

Looks like when we merged the thanos work we missed that the grafana datasource for prometheus got replaced with this one:

https://github.com/redhat-cop/pelorus/blob/master/charts/deploy/templates/grafana-thanos-datasource.yaml

We'll need to handle the cases more intelligently.