Rudder

We want to provide a rudder global (and perhaps for group) property with important default properties. Not having them would broke rudder. So we don't want an user to be able to change/delete them.

We propose to use the same `provider` system than for node properties, with the following new set of rules addapted to inheritance:

- for a given object (global, group, node), only the same provider can change an existing property appart if that provider is the default one in which case any provider can update the property and set themself as providier (ie: same rule as before, but for group and global in addition to node),
- provider don't change semantic of inheritance and overriding: a property with a provider different from default can be overriden by any provider, and at each level, fully inherited properties (ie virtual one, which are not really on that object) get an "inherited" provider.