Enable prepuller

I disabled it https://github.com/IDR/k8s-analysis-deploy/pull/8 because it was regularly failing in staging and blocking deployments. There are several upstream issues and PRs which mean this may work again in the latest version of zero-to-jupyterhub:
- https://github.com/jupyterhub/zero-to-jupyterhub-k8s/issues/726
- https://github.com/jupyterhub/zero-to-jupyterhub-k8s/issues/735