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. We also
provide a standard way of retrieving a plugins options for loading via
other mechanisms.

Blueprint: standard-client-params
Change-Id: I353b26a1ffc04a20666e76f5bd2f1e6d7c19a22d
Merge "Plugin loading from config objects"