Grafana: Use wildcard for all on cluster overview

Instead of listing all clusters manually in the query, i.e like:

```
cluster=~"(local-cluster|simulated-managed-cluster-1|simulated-managed-cluster-1-1|simulated-managed-cluster-1-10|simulated-managed-cluster-1-2|simulated-managed-cluster-1-3..."
```

We set it to `".+"` simplifying the query significantly.

Signed-off-by: Jacob Baungard Hansen <[email protected]>