adminutils: fix for global admin instance

* Fixes an issue where Flask-Admin instance was created globally and
  not per Flask application, causing problems when running e.g unit
  tests where each tests gets a new application.

Signed-off-by: Lars Holm Nielsen <lars.holm.nielsen@cern.ch>
Signed-off-by: Jiri Kuncar <jiri.kuncar@cern.ch>