Switch to oslo.config fixture

The fixture from oslo-incubator has moved to oslo.config,
we need to use this from now on. The copy of the fixture
in nova/openstack/common/* will be removed in a subsequent
review in this series.

Change-Id: I2d91db8c1535ca60f59f9d55c9d19de3e84dc40b
Merge "Switch to oslo.config fixture"