Add support for multiple authentication plugins

[Lance Bragstad]

KSA requires a specific authentication plugin in order to communicate with the identity server and prove a user's identity. For multi-factor like flows, we need to ability to specify multiple authentication plugins. This discussion came up in IRC [0]. Adding support for this will make multi-factor authentication flows accessible to clients (like python-openstackclient) to consume.

[0] http://eavesdrop.openstack.org/irclogs/%23openstack-keystone/%23openstack-keystone.2017-08-08.log.html#t2017-08-08T15:01:00