Fix preface when using ADoc [preface] role

[tomschr]

• Copy a real ADoc preface as DocBook preface
• Only use the variablelist in an empty preface[title='']. This goes into an inserted in

This separates the abstarct info and the real preface info. From articles/ai-requirements.adoc:

[abstract]
--
WHAT?::
   Hardware, software and network
WHY?::
  To ensure efficient operation of {productname}.

----

[preface]
== Preface
This article describes the software, hardware and networking requirements for the cluster nodes where you plan to install {productname}.