Ironic driver persistent warn msg when running only a node per conductor group

[Belmiro Moreira]

```
2021-05-07 13:55:12.570 3142 WARNING nova.virt.ironic.driver [req-bcca8fbe-3293-4d85-a3a3-a07328d91c17 - - - - -] This compute service (XXXXXXX) is the only service present in the [ironic]/peer_list option. Are you sure this should not include more hosts?
```

The decision about the number of compute nodes behind each conductor group depends in the deployment architecture and risk tolerance.

For deployments that decided to only run one compute node per conductor group they get the above msg in the logs every periodic task cycle.

It's good that Nova points that this can be an issue, but the frequency really "pollutes" the logs for all the operators that made a conscious decision.

I propose to move the log level from warn to debug.

In debug operators will continue to have this message. Usually operators run in debug mode when debugging issues or in the deployment phase.

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.opendev.org/c/openstack/nova/+/790248