This should not be a breaking change, because examples/vars.yml
explicitly enables Traefik.
Someone needs to have gone out of their way to delete these variables
to rely on the default specified in `playbook_base`.
Certain environments (like https://libreelec.tv/) do not have
`useradd`/`groupadd` tools, so it's important to be able to avoid
calling them, especially for no good reason.