Invalid filter when selecting a single instance

When selecting a single instance, the query passed to gnocchi seems to be incorrect.

```
params: {filter: "instance_id in cb5f8625-155c-4528-a51e-
"Invalid input: expected a list for dictionary value @ data['in']['instance_id']”
```

We're using this dashboard: https://grafana.com/dashboards/4569. The plugin could probably ensure that this is actually handled as a list.