oauth2server: redis configuration fix

* Fixes issue with Flask-OAuthlib not using the value of
  CACHE_REDIS_URL configuration variable.

Signed-off-by: Lars Holm Nielsen <lars.holm.nielsen@cern.ch>
celery: email address for errors

* Changes default Celery email address for errors to always be
  CFG_SITE_ADMIN_EMAIL.

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