User's don't know what a plugin is.
* Internally, os_auth_type and/or auth_type represents what the
user supplied.
* auth_plugin_name is the name of the selected plugin
* auth_plugin is the actual plugin object
Plugin selection process:
* if --os-auth-type is supplied:
* if it matches against an available plugin, done
* (if it can map to an availble plugin type, done; TODO in a followup)
* if --os-auth-type is not supplied:
* if --os-url and --os-token are supplied, select 'token_endpoint'
* if --os-username supplied, select identity_api_version + 'password'
* if --os-token supplied, select identity_api_version + 'token'
Change-Id: Ice4535214e311ebf924087cf77f6d84d76f5f3ee