Comment 2 for bug 1938155

If I modify the YAML file to specify focal as the series, the overlay works:

```
applications:
  prometheus2:
    charm: prometheus2
    num_units: 1
  node-exporter:
    charm: prometheus-node-exporter
    series: focal
relations:
  - - prometheus2:scrape
    - node-exporter:prometheus
```

But does not work with CentOS7.