Removed duplication with list_user_ids_for_project

Keystone list_user_ids_for_project has the possibility of returning
duplicate users if the user has more than one role in the project.
list_user_ids_for_project queries the assignment table which has
one entry for every role the user has but will now remove
duplicate combinations of target_id and actor_id. This impacts
V2 calls for tenants.list_users(<tenant_id>) and
users.list(tenant_id=<tenant_id>).

Change-Id: I9a1e5b944c2c627138851976e81d405f54fae1c9
Closes-Bug: #1308218
(cherry picked from commit d6cb7043cb47f5e73b61635af596736314441275)
Merge "Removed duplication with list_user_ids_for_project" into stable/icehouse