host -> host_ip[0]
Remove deprecated configuration notification_topics.

WARNING oslo_config.cfg [-] Deprecated: Option "notification_topics"
from group "DEFAULT" is deprecated. Use option "topics" from
group "oslo_messaging_notifications".

[0]https://docs.openstack.org/cyborg/latest/configuration/sample-config.html

Change-Id: Ia5d53fb60d34c1509c6cdb905cbd0a93dd1c8b3d