Fix duplicate keystone auth_token config options

Currently, Mistral registers the configuration options under
keystoneclient middleware auth_token twice: by default when the
keystoneclient.middleware.auth_token module is loaded and in mistral.config
under a separate keystone group. This change removes the keystone group
in mistral.config, modifies API to use the default opts registered under
keystone_authtoken, and adds unit tests to ensure the keystone middleware
is called when auth is enabled.

Change-Id: I18948f7fa8b93b458335eb08176427a75c568873
Implements: blueprint mistral-config-cleanup