Comment 3 for bug 1673375

Looking at logstash:

http://logstash.openstack.org/#dashboard/file/logstash.json?query=message%3A%5C%22_send_notification%5C%22%20AND%20message%3A%5C%22ValueError%3A%20Circular%20reference%20detected%5C%22&from=7d

This seems to only show up on that aggregate negative test, and it's probably this error notification wrapper:

https://github.com/openstack/nova/blob/2380659e358770a3f36253b93a112b9779a23958/nova/compute/api.py#L4602

The AggregateHostExists duplicate error comes out of the call to "aggregate.add_host(host_name)".

I'm not sure what's breaking in the actual notification driver. It also looks like this is happening on non-py35 jobs too.