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.

Change-Id: I1e83280b2f76f16041ed8d5ed598db70210112bd
Session loading from CLI options

We will want this to standardize session loading amongst the various
CLIs.

Change-Id: Icc740db6d471a0953b7946e00e6317802b6d2255
Plugin loading from config objects

Provide a pattern for auth plugins to load themselves from a config
object. The first user of this will be auth_token middleware however it
is not likely to be the only user.

By doing this in an exportable way we are defining a single config file
format for specifying how to load a plugin for all services.

Loading from CONF is just one of the ways plugins might be loaded.
The decision not to expose params as oslo.config options is on
purpose as there is cliff and other loading mechanisms besides
oslo.config we need to support.

Blueprint: plugin-params
Change-Id: I353b26a1ffc04a20666e76f5bd2f1e6d7c19a22d