Fix cinder Debian install procedure

This patch adds links to the "Respond to the prompts for" steps,
and removes the manual edition of cinder.conf for the
keystone_authtoken (as this is handled by Debconf).

This is a backport of: https://review.openstack.org/#/c/97158/

Change-Id: I4ef97b63dcd735077978ec26cea2acc6f8b36f82