Users are declared in one comprehensive file,
regardless where they have an account.
They are then included in groups.
Each forest, ie a set of nodes (servers), define the group they want.
On each node of this forest, users belonging to one group for this
forest will be created (and groups too).
Furthermore, some groups can be added in shellgroups_ubiquity to be
created on each forest.
Provide a forest execution module to get a list of users or groups.