Remove reliance on import order of oslo.db mods

This change fixes a couple of import statements that rely on some import
order and a bit of magic to make the oslo.db.options module visible
under the oslo.db package without importing it explicitly.

The old version of the imports relies on Python's import machinery to
attach sub-modules to packages after import (see
http://paste.openstack.org/show/148711/). It is safer to explicitly
import the module we need, to ensure that we are not relying on
side-effects of importing other modules to configure global state for
the application.

Change-Id: Ia2772030915061ff373e7b7dc77f161146b56e59
Merge "Remove reliance on import order of oslo.db mods"