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
