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
a7ee3ac5c8