Session loading from conf

Allow loading session objects from oslo.config. We want a generic way to
do this for auth_token middleware and for servers creating session
objects for inter-service communication.

DocImpact: This is the first step in standardizing all the config
options across projects. There are no changes to the config options that
keystoneclient actually consumes in this review.

Implements: blueprint standard-client-params
Change-Id: I1e83280b2f76f16041ed8d5ed598db70210112bd