Add role assignments rbac tests to identity v3.

Add role assignments rbac tests to identity v3. These
tests cover the policy actions here [0]:
  - identity:list_role_assignments
  - identity:list_role_assignments_for_tree

[0] https://github.com/openstack/keystone/blob/master/keystone/common/policies/role_assignment.py

Change-Id: I7bdeedf51f77ba008dbad74c32c668eb0cab8aac
Partially-Implements: blueprint initial-tests-identity
2 files changed