Follows-up from T127771. See also T161675.
The Horizon interface allows enabling roles on instances. Both via the per-instance "Puppet configuration" tab, and via the project-wide configuration page based on instance name prefixes (somewhat similar to what we do in site.pp for production).
The problem is that while role::deployment_server is applied to tin.deployment-prep (Beta Cluster), the Hiera values from role/common/deployment_server.yaml are not.
It seems for the most part, things specific to an individual cluster have their own Hiera yaml file and don't use the role file, so it seems safe to include these by default in Beta Cluster, with per-cluster overrides as we do already (eqiad, codfw, labs, etc.)
It should be possible to write something in Puppet for production in a way that also works on Beta by default (if it doesn't need Beta-specific overrides). Right now that is not the case because scap::sources needs to be kept in sync via deployment-prep's own Yaml file.