[TrivialFix] Use _override_role in rbac_rule_validation

This PS changes switch_role usage to _override_role usage in
rbac_rule_validation. This is a temporary change while switch_role
goes through a deprecation phase. This code change should have been
made earlier in:

    I670fba358bf321eae0d22d18cea6d2f530f00716

Partially Implements: blueprint rbac-utils-contextmanager

Change-Id: I7154b7bf9440aaef69c8dae9b6643f956564369f
1 file changed