Add a job for seed deployment

Adds a new CI job, kayobe-seed-centos, that deploys a single VM as a
seed host. The VM provided by Zuul is configured as the seed, rather
than a seed hypervisor that provisions a seed VM using nested virt.

This approach should be more reliable, faster, and easier to get
working, at the expense of not testing the seed hypervisor code paths.

Currently the seed services are deployed, but not tested. A future
change could test discovery and provisioning of an overcloud host using
the seed.

Change-Id: Iefe16215af6f1a9fb8ec78094cd6bd76a82a040a
Story: 2001655
Task: 6683