Fix get project users when no user exists

The server would respond with a 404 Not Found if the client
requested a list of the users with a role on the project and any
of the users didn't exist in the identity backend. The user may
not exist because the server doesn't enforce that users exist when
granting roles (because of federation, the users may not exist in
the identity backend).

It's inappropriate to respond with a 404 Not Found for the
/v2.0/tenants/{tenant_id}/users resource because the resource
actually does exist.

The server will now respond with a 200 OK and will just not include
users that don't exist.

bp no-check-id

Change-Id: Idf95836bf54470d91032d2c3ace970ca471cf25d