Allow for skipping admin roles loading on context creation

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
Merge "Allow for skipping admin roles loading on context creation"