This partially reverts commit 47bbb96b29ab30764d6220cdd43e63a1d2072533
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.
Change-Id: I6bc8da2b8da903e09b97df8cd95c68a562c11db9
f9d9afcfba