general: legacy directory pre-creation

* Ensures legacy directories are pre-created upon legacy extension
  `setup_app()` call.  (closes #1789)

Signed-off-by: Samuele Kaplun <samuele.kaplun@cern.ch>
Reviewed-by: Yoan Blanc <yoan.blanc@cern.ch>
Reviewed-by: Jiri Kuncar <jiri.kuncar@cern.ch>