Init step postRun() introduced for 2.9-SNAPSHOT compliance.

Gerrit 2.9 includes the ability for plugins to execute their
initialisation steps after the initial run with a fully populated
DB. We don't use yet this capability but we need anyway to implement
the empty method to be able to load the init step.

Change-Id: Ibc621ea5421c9fb1580421ba3168a2eeb2ad983a