Add oslo.config to test requirements
Add oslo.config to test requirements

The cross-testing scripts in oslotest require oslo.config, but can't
depend on it directly because that introduces a circular dependency,
so add it to the list of test dependencies for now like we do in other
projects.

Change-Id: Ibd1ad7a26e8be07797a156d6ffb312f33c3c89f5

magic0704/oslo.utils,openstack/oslo.utils