Allow the user to create an inference rule between
two roles. The first, called the prior role
is the role explicitly assigned to an individual.
The second, called the implied role, is one that
the user gets implicitly. For example:
Role B implies Role A.
User X is assigned Role B.
Therefore User X also assigned Role A.
The management and maintenance of the rules is
performed in the Keystone server.
Change-Id: If547c2f16e812bc7fffd742ec37e6a26011f3185
8cd3e258c5