Install federation bits for keystone to devstack.

Although we decided not to have an external repository, for the sake of POC
the plugin lives here. When the code will be sent for review, it will live
in the main keystone repository. ``devstack-plugin-federation-test`` will
be changed to ``keystone``, no other changes will be required.

Use this local.conf for a quick test:

[[local|localrc]]
ADMIN_PASSWORD=secret
DATABASE_PASSWORD=$ADMIN_PASSWORD
RABBIT_PASSWORD=$ADMIN_PASSWORD
SERVICE_PASSWORD=$ADMIN_PASSWORD

HOST_IP=192.168.56.101
disable_all_services
enable_service key mysql
ENABLE_IDENTITY_V2=False
enable_plugin devstack-plugin-federation-test https://github.com/bretonium/devstack-plugin-federation-test