In a previous version I could explicitly set parameter '::foreman::user_groups' to the empty array, thus preventing the automatic addition of a 'puppet' group entry. This is no longer the case after a logic change in foreman::config.
However, this logic change seems to add a 'puppet' group entry even if the host (where I wish to install foreman) is only a puppet client (it happens to be the case that in my environment foreman and puppet compilation masters all run on different hosts). Since puppet client installation do not configure a 'puppet' group, the foreman::config manifest fails when creating the 'foreman' user.