Bug 1216866
There are cases in which an admin context is created only to grab a db
session and ensure no tenant filters are applied in _model_query. In
these cases evaluating the policy engine for grabbing admin roles is not
necessary, and can cause unexpected and serious issues if the context is
grabbed before all the extensions are loaded.
Change-Id: I0cbf4b51ca1286373c16eb907840a32f4b8190c6