Fix Root Access hack in Ubuntu

[srbarrios]

We could not catch this issue uyuni-project/uyuni#5521 due to the hack we do:

https://github.com/uyuni-project/sumaform/blob/master/backend_modules/aws/host/user_data.yaml#L191-L194

Instead of that, we should do exactly as the documentation propose:
https://documentation.suse.com/suma/4.3/en/suse-manager/client-configuration/clients-ubuntu.html#_root_access