Use config fixture from oslo.config

With the release of oslo.config 1.4.0.0a1, oslo.config
contains the config fixture. python-keystoneclient was using
the config fixture in oslo-incubator's fixture module. With
this change python-keystoneclient is using the config fixture
in oslo.config

Change-Id: Id03efdfbe8893c48a9ad65f0cf59e1dbae03dc47