CI: separate image builds into a non-voting job

Disk and container image builds tend to be fairly unreliable.
With 3 voting seed jobs all building images, this can introduce
instability into the CI jobs.

This change adds a non-voting kayobe-seed-images-centos8s job, which
does the following:

* Builds IPA images
* Builds an overcloud host image
* Builds a base container image

Similar Rocky and Ubuntu jobs are added to the experimental pipeline,
and may be run by commenting 'check experimental' in gerrit.

The existing kayobe-seed-* jobs no longer build images.

Change-Id: Idecda342f3ab86733e8d59061458d44af834dbb0