Rudder

We want to have group properties like we have node properties.

The difficulty if the interaction with other properties:
- node property with the same name as a group property would override the value
- 2 group properties with the same name on applying on the same node must have a way to tell which one is the one to take

One solution would be to have a hierarchy of group.

I think a better one would be to have a priority per group and chose the property based on the priority.

But we could probably have something more obvious for the user.

Update: we have think to something that can fly:

- if two groups define the same key, we fail generation except if one group is a subgroup of an other.
- nodes automatically get all groups properties the are in for variable expansion (but that's not "their" property, ie they can directly modify it, nor these property can be used to define groups).