gitlab_provision: init

Allows to provision users and map them to groups with some role.

The configuration supports 3 levels of groups.

Since a TF resource cannot reference itself (cycle) we need to define different resources for 1st level groups, 2nd level groups etc... so that a 2nd level group can reference a 1st level group.

For group_membership resources we make sure the key is unique so it includes the path + name of the group.