Some services (such as telemetry) actually consume the notifications.
If one deploys a service that listens on the same queue as telemetry,
there will be race-conditions with these services and one will not get
the notifications that are expected at points.
To address this, one sets a different topic and consumes from there.
This is not possible with versioned notifications at the moment. And, as
services move towards using them, the same need will arise.
This adds a configuration option to Ironic enabling the configuration
of topics for this notifier.
A similar change was introduced in nova:
https://review.openstack.org/#/c/444947/
Change-Id: Ib75feac0979d0094cb137abb13b0fe0ff4576eee
Story: 2004735
Task: 28788
0e81ec00ce