monitor ganeti orphaned disks and other warnings/errors from gnt-cluster verify

In #41639 (closed) we had the intention of setting up monitoring for ganeti. We already have some ganeti-related metrics in tpa_ganeti_*. There's an exporter https://github.com/ganeti/prometheus-ganeti-exporter that can get us more information.

We possibly don't want to alert on the hbal score.

But we might want to alert on warnings and errors from gnt-cluster verify.