Change datetime.now() to timeutils.utcnow() from oslo_utils

We use an UTC time to avoid the difference with time zones.

Change-Id: I15aa3b5d3337b90ccdcc6c4ac5d3c7d78108fe21
Related-Bug: #1288979

cinder/db/sqlalchemy/api.py

@@ -3571,7 +3571,7 @@ def purge_deleted_rows(context, age_in_days):
         LOG.info(_LI('Purging deleted rows older than age=%(age)d days '
                      'from table=%(table)s'), {'age': age_in_days,
                                                'table': table})
-        deleted_age = dt.datetime.now() - dt.timedelta(days=age_in_days)
+        deleted_age = timeutils.utcnow() - dt.timedelta(days=age_in_days)
         try:
             with session.begin():
                 result = session.execute(