CentOS 8: Add seed and overcloud CI jobs, drop CentOS 7 jobs

CentOS 8 removes interfaces from their bridge during ifdown, and removes
the bridge if there are no interfaces left. When Kayobe bounces veth
links plugged into the bridge, it causes the bridge which has the IP we
are using for SSH to be removed. Use a dummy interface in CI to avoid
this problem.

Kolla-ansible has dropped all CentOS 7 jobs on master now, and prechecks
only allow CentOS 7 hosts. Drop all CentOS 7 jobs. We will have to run
without upgrade jobs in place until Train supports CentOS 8.

Depends-On: https://review.opendev.org/695881

Change-Id: I7c1a885b36445e33d4db1b1c8533db28a644b4a1
Story: 2006574
Task: 38870