bootstrap-vz 0.9.11+20180121git-1 doesn't know about Buster. It also tries to install Puppet from puppetlabs.com APT repository.
I have a hacked bootstrap-vz version that was able to build an image successfully. Pending tests.
However, it seems bootstrap-vz is up for adoption. I have some experience using packer to create KVM images and I think that would be more future-proof at this point but I would like to first see if there is any hope for bootstrap-vz just so we don't spend too much time on this right now.
Our Puppet repository isn't prepared for Buster so this experiment will have to stop here for now.
In any case, the image I created booted fine but failed to run Puppet, as expected.
I used cloud-bootstrapvz-buster-tmp.openstack.eqiad.wmflabs to build it. It has a hacked up version of bootstrap-vz that knows about Buster.
I will leave the image debian-10.0-buster-test deactivated in OpenStack. Attached is the console.log for the first run.
You need to use facter 2, it's built in buster-wikimedia, then it should work fine, that's also what we do in production. But at this point I don't recommend to make buster images available to the WMCS public, let's wait until it's released.
I've merged buster patches upstream, and built a +wmf release with those patches. It's in repropro, named 'python-bootstrap-vz'. I presume that eventually we can go back to using the upstream package if it's rebuilt for future buster releases.
There's a build host in the 'openstack' project which can build buster base images which work.