Add auth_token settings to cinder.conf.sample.

Updates our example config file so that it includes configuration
settings for the auth_token middleware.

A step towards using cinder.conf instead of the api-paste.ini config file
for authtoken configuration.

Once https://review.openstack.org/#/c/52259/ lands I'll push another
review which removes the settings from api-paste.ini (which takes priority).

Related-Bug #1240753

Change-Id: I6636d33ee522757145ac97fc354324a8b9379700
Merge "Add auth_token settings to cinder.conf.sample."