Correct API for listing user roles & role assignments

The v3 API specification listed two api calls relating to roles that
are, in fact, un-implemented in the Grizzly code, namely:

GET /users/{user_id}/roles
GET /roles/{role_id}/users

These are replaced with a more generalized call for listing role
assignments, allowing for filtering based on domain, project, role,
user and group.

bp get-role-assignments
Fixes bug #1183430
Fixes bug #1183433

Change-Id: I8d3e61aab053f230fdc277f7847a58d834694ac4
Merge "Correct API for listing user roles & role assignments"