ConfigUpgrader needs to be more generic

[Florian Boucault]

As of today, for each new RC, release, etc. the config upgrader needs to be updated by:
 - adding configuration file upgrade code that handles n-1 -> n upgrade
 - add a call to that code in the n-1 -> n, n-2 -> n, n-3 -> n cases and so on and so on

This needs to be made more generic so that just the first step is required and all possible upgrade paths to n are covered.

[Philippe Normand (philn)]

See [5321]