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>