Calculate timestamps within DB, not client

[Yikun Jiang]

Calculating the timestamp in the clients can lead to problems because network latency is variable, clients experience slightly different clock drift.

SQLAlchemy allows us to do this by passing func.now() which tells the DB to calculate the timestamp itself

So, in order to make the updated_at and created_at more accurate, we should calculate the timestamp in db.[1]

[1] https://github.com/openstack/oslo.db/blob/3ef34da/oslo_db/sqlalchemy/models.py#L138-L139
[2] https://stackoverflow.com/questions/12154129/how-can-i-automatically-populate-sqlalchemy-database-fields-flask-sqlalchemy
[3] https://stackoverflow.com/questions/13370317/sqlalchemy-default-datetime
[4] http://docs.sqlalchemy.org/en/latest/core/defaults.html#client-invoked-sql-expressions

[OpenStack Infra (hudson-openstack)]

Change abandoned by Stephen Finucane (<email address hidden>) on branch: master
Review: https://review.opendev.org/592304