CI: convert vagrant jobs to kubevirt

Vagrant jobs needs a big cache which makes them slow / sometimes stuck
completely. Using the kubevirt provisionning playbook is now
significantly faster, so do just that.

Having only one provisionner in CI will also allows us to remove some of
the custom runners executors we use for vagrant, and more generally
reduce the CI maintenance.

Our kubevirt CI platform does not support ivp6 yet, so we keep the
relevant jobs in vagrant, but we'll migrate them as well as soon as
possible.