Adds the kayobe-seed-vm-centos8 CI job to configure the Zuul VM as a
seed hypervisor, and use nested virt to provision a seed VM.  This
ensures that the seed hypervisor code paths are tested.

The job uses a Cirros image for the seed VM rather than the usual CentOS
cloud image. This is to reduce bandwidth required to download the image.
It does mean that the resulting seed VM cannot be used as a seed, but
nested virt would make this slow and unreliable anyway. Cirros does not
load cdrom drivers by default, so we add the configdrive as a disk
rather than a cdrom device.

Depends-On: https://review.opendev.org/617161

Change-Id: I2268a1ddf9a2870c713f32a40689e1686365aabd
Story: 2001655
Task: 6683