Use models.TimestampMixin from oslo.db

Since `ironic` and `oslo.db` uses `timeutils` from one place,
we can remove a temporary hack and re-use `TimestampMixin`
from common db code.

Change-Id: I48b2ce26484f1283ec65ae4f8dda57d962a8a875
Closes-Bug: #1354443
Merge "Use models.TimestampMixin from oslo.db"