Change to allow read of service names from doc file

- Clouddocs-maven-plugin reads service names
  and links to related API ref pages
  from the api-ref/src/docbkx/preface.xml file
- Added an xml:id to the itemizedlist
  in the preface.xml file to identify it as
  the service names/links source
- Added preface to every API reference page
  so that each page can auto-generate the
  sidebar menu from the itemizedlist in
  the preface

Partial-Bug: #1267174

Change-Id: I8871c804c2c93f4f7625fc92ed0448de40ccb751
author: diane fleming
Merge "Change to allow read of service names from doc file"