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

[fanf]

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.

[ncharles]

This looks good. Could you check that it does not use too much memory ?

[ncharles]

I tested it with a limited test set of 50 nodes
From this limited test, i gathered that:

• memory use does not seem different
• fetching node information is about 200-250 ms slower (so 5 ms per inventory it seems)

I would need to test with more nodes to confirm these data

[fanf]

The feature was F-review.
It is really low impact and needed for test by some user, so could be merge asap.