Assignment Manager Cleanup

The assignment manager [1] contains several methods to list actors, roles and targets based on the role assignments.
They implement assignment inheritance and grouping expansion logic.

The list_role_assignments method correctly implement both inherited assignments and grouping logic.
Then, instead of repeating code, those methods should call it and then get the information need to return.

For example, list_projects_for_user should list effective role assignments for the given users and then return the distinct projects that user has a role on.

[1] https://github.com/openstack/keystone/blob/master/keystone/assignment/core.py

This was discussed during the Keystone IRC meeting on 2/14/15, and it was agreed that this does not need a spec.