Add auth config for neutron metadata proxy

 * Without auth config, the proxy will return 500 errors on
   cloud-init requests, which will cause VM connectivity
   checks to fail.
 * A cleaner fix would be for the metadata proxy to reuse the
   configuration from the keystone_authtoken section of
   neutron.conf, but I chose the easier route because of a
   pending switch from REST to RPC communication (RPC won't
   need the auth config).
 * Fixes bug 1210664

Change-Id: Iaa3c74f5ada2404119c44c8cbdad380eda158f66