Rudder

This is the main entry point for the feature that will allow to have software updates for a node.
It encompasses at least:
- change in agent inventory command (in fusion inventory) to add software updates,
- change in parsing of inventories by webapp to understand these tags,
- change in persistance to have these information saved in LDAP for each nodes,
- change in Node object and API to expose these informations.

Different tasks will have different sub-issues.