Remove Notification, NotificationHistory status labels:

Replace labels by adding a key kwarg in the model for status.

We still need this as sqlalchemy attmempts to look for `notification_status`
on the model (Notification/NotificationHistory). To achieve true ORM mapping
(map status -> notification_status) we need the key kwarg.

More here:
http://docs.sqlalchemy.org/en/latest/core/metadata.html#sqlalchemy.schema.Column#key

app/dao/statistics_dao.py

@@ -31,7 +31,7 @@ def timeout_job_counts(notifications_type, timeout_start):
     results = db.session.query(
         JobStatistics.job_id.label('job_id'),
         func.count(Notification.status).label('count'),
-        Notification.status.label('status')
+        Notification.status
     ).filter(
         Notification.notification_type == notifications_type,
         JobStatistics.job_id == Notification.job_id,