Adding the callback mechanism to neutron-lib. Added code, test, and
devref. Added additional test coverage. Added missing __init__.py
files so tests are run. Needed oslo.log added to requirements.
Once this is upstreamed, we can modify the callback mechanism.
Change-Id: Ib16f3942e8ac2ddbfc8ff6919863ec9ad197e5b6
Implements: blueprint neutron-lib
3d41d2e8e5