Fixes #5225: allows to use ${rudder.node.env.VAR} syntax to get env variable in directives

The main idea is to add NodeInventory in the context of rudder variable interpretation. 

The big problem is that doing so, we need to load all node inventories before hand, perhaps for nothing. 

So we need to check the time and space take by that approach on a non trivial node set. 
That pull request should wait that test result before being merged.