stop validating user IDs on role assignment operations

Keystone is proposing to change it's behavior regarding user & group ID
validation on role assignment operations in order to establish full
isolation between the assignment and identity interfaces, which were
split apart during havana.

Change-Id: Id0549d6b26f348f6029c238adfcb4d8dc47be316
Implements: bp no-check-id
Merge "stop validating user IDs on role assignment operations"