Add trust users to AccessInfo and fixture

A trust should always contain a trustee_user_id and a trustor_user_id.
Expose these values via AccessInfo if available.

Change-Id: Ic46a44300e6bf8aa694f1543d470c16fcac643fc
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
Allow loading auth plugins from CLI

With a standard definition of auth plugin options we should be able to
load and use those plugins from command line applications. Provide a
mechanism to register argparse parameters and load from them.

Blueprint: standard-client-params
Change-Id: I5d9904fa885602aaaef7a9e0afd4bd6bbfca3f07
Merge "Plugin loading from config objects"
Merge "Add trust users to AccessInfo and fixture"
Merge "Allow loading auth plugins from CLI"