Add information about pluggable auth handlers

fixes bug 1084497

This patch adds a new file and includes it from the common
keystone chapter. Grizzly brings the ability to use the auth
handlers from apache-httpd, and this change takes docs from
the keystone repo to make them available for operators.

Change-Id: Ife00dfb48ec717598b545ee1ffc1ccccaa89b6ad
Merge "Add information about pluggable auth handlers"