This partially reverts commit
which triggered a retry on vexxhost clouds.
The issue was introduced in Ie8fd965165e8d347d27528a2c16d0647e412ccdc,
which applied some fixes for CentOS 8.3, and inadvertently removed
the Tenks variable that forces the use of qemu for 'bare metal' VMs.
This lead to autodetection of KVM, which does not work well when nested
in all CI cloud providers.
This change fixes the issue by forcing the use of qemu for the overcloud
once more. It also adds a similar option for the seed VM job.