Add user-data and config-drive information to End User Guide

These three files have not been included in any of the current guides
and we have broken references to them:
common/section_user-data.xml
common/section_nova_cli_userdata.xml
user-guide/section_nova_cli_config-drive.xml

This patch adds them to the End User Guide and fixes references to
them.

Note: I'm not 100 per cent sure that the End User Guide is the right
place to add these. Please double check and advise.

Partial-Bug: #1225093

Change-Id: I12538a6c8f8f84e5a469a8b63b1dd1b71f44d21a
Merge "Add user-data and config-drive information to End User Guide"