Fix mocking of utcnow() for model datetime cols

Now when we are going to deprecate timeutils.set_time_override()
and use mock instead, it easy to hit the following gotcha: when
testr does tests discovering, it imports all modules within a given
directory and it might happen that common.db.sqlalchemy.models
is imported before timeutils.utcnow() is mocked in a test case.
In this case the SQLAlchemy model will save the reference to the
original timeutils.utcnow() function, which will be called later
when the model will be saved to a DB, if created_at or updated_at
value aren't provided.

In order to overcome this we can defer resolution of timeutils.utcnow
reference by passing lambda functions to SA Column __init__().

Related-Bug: #1266962

Change-Id: I7adce90eacb4a3f334d77da7b4a176c31ff818ed

openstack/common/db/sqlalchemy/models.py

@@ -102,8 +102,8 @@ class ModelBase(object):
 
 
 class TimestampMixin(object):
-    created_at = Column(DateTime, default=timeutils.utcnow)
-    updated_at = Column(DateTime, onupdate=timeutils.utcnow)
+    created_at = Column(DateTime, default=lambda: timeutils.utcnow())
+    updated_at = Column(DateTime, onupdate=lambda: timeutils.utcnow())
 
 
 class SoftDeleteMixin(object):