perf: optimize overview dashboard queries

[stehessel]

Optimize a few of the Grafana queries in the cluster overview dashboard:

• Only show top 5/10 values dashboards that filter by namespace. This reduces the number of time series that are queried, and also makes the widgets less cluttered. Note that the actual number of different namespaces may still exceed 5/10 if the top namespaces changed during the dashboard time window.
• Filter out useless data in the tenant table.
• Enable pagination for the scanner table.

[janisz]

Why indentation has changed?

[stehessel]

There was a mix between tabs and spaces before. My guess is that somebody edited the .json by hand and used an editor which did that (probably me). Now it's just the .json generated by Grafana, which is all spaces. Or Grafana changed the tabs -> spaces, that might also be the case since we updated the Grafana version at some point.