If an AllowGroups directive is found or when using Debian 9+,
we use the AllowGroups directive and comment AllowUsers that may be
already present.
When adding a user, we make sure that the allowed group exists
and the use is in that group, to be sure that at least this user
is allowed to connect.
In other situations, we use the AllowUsers directive.
* rename admin-users to evolinux-users
* splitting the "sudo" part for users between jessie and stretch
* with stretch, the sudo group is customizable and properly configured
* import evolinux-users role from evolinux-base at proper time
to ensure ssh connections are possible for other users before
cutting root's access
* evomaintenance is also included in evolinux-base to have it available
when users are created
Jérémy Lecour