Update neutron-lib callbacks with the latest from neutron
in preparation for updates to the callback notification interface.
The callback updates also require a few module level functions
in the db package.
A few additional unit tests are also added for coverage as is
python docstrings for the public API contained herein.
Change-Id: I74effcc4a564a92c418132042bbdd5bed22c4950
Here we add the context module from Neutron, although it is
currently marked as private because it will undergo changes
related to the ongoing enginefacade and Keystone V3 work.
We also add some of the roles management methods from
Neutron's policy module. These are required for the initial
DB patches that follow this one.
Partially implements: Blueprint neutron-lib
Change-Id: I77f8a05d6e3167f3096ad637d124e47ac39a83df
