Make db/api strip timezones for datetimes

Before objects, we always used TZ-naive dateteimes, but actually
in the UTC timezone. With objects, we use TZ-aware datetimes with
UTC as the zone. This makes the DB api strip out the timezone
before passing them to sqlalchemy, which expects naive ones.

Related to blueprint compute-api-objects

Change-Id: Id28378c6c68cabcc677bf8cf7b47a4446c397505
Merge "Make db/api strip timezones for datetimes"