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__().