Update oslo-incubator fixture to 81c478

This updates the oslo-incubator fixture module level to commit
hash 81c478

 $ git checkout 81c478
 $ python update.py --nodeps --base keystone \
     --dest-dir ../keystone --modules fixture

The changes include a new feature in the Config fixture to
register new config options.

Change-Id: Ia01062d160b6daaa12c87cc77166017be3fa2f30
Merge "Update oslo-incubator fixture to 81c478"