Use timeutils from one place

Usage of timeutils from different places (from oslo.db and from ironic)
can produce errors (for example, during porting oslo.db to using
oslo.utils, see https://review.openstack.org/#/c/111701/ )

So we need to remove all imports 'oslo.db.openstack.common.timeutils' and
don't use `oslo.db.sqlalchemy.models.TimestampMixin`, until ironic and
oslo.db will use timeutils from one place(from oslo.utils).

Related-Bug: #1354443

Change-Id: I53f519570df3351b6acf019d89f67b43d8b637da
Merge "Use timeutils from one place"