CI: Zun jobs

- Test Zun on CentOS too
- Make etcd change also trigger Zun jobs (like kuryr and zun)
- Test multinode Zun deployments instead of AIO
  (more likely to break)
- In Zun scenario, stop configuring docker for legacy swarm mode
  (Zun is no swarm)
- Separate test-zun.sh testing script
- Show appcontainer to see which node it has been started on

Change-Id: I289b1009fe00aedb9b78cbd83298b14da5fd9670
Depends-On: https://review.opendev.org/676736


Signed-off-by: Radosław Piliszek <radoslaw.piliszek@gmail.com>