Add tests and doc entry for request.environ[reseller_request]

The recent account_quotas (https://review.openstack.org/23434)
patch added a new setting request.environ[reseller_request].
This patch adds tests for tempauth and keystoneauth as well as
an updated overview_auth.rst.

Change-Id: Icdb7ec9948ae7424b0721fc51a143782b2fdc5a6
Merge "Add tests and doc entry for request.environ[reseller_request]"