Adding documentation for proxy domain usage

Changes
* creating an advanced configuration guide
* adding a link for the advanced config guide to index
* changing the notification config heading to subsection
* adding labels to diskimagebuilder and hadoop-swift documents

Partial-Implements: blueprint edp-swift-trust-authentication
Change-Id: Ie3a3cf0b470ac67670251a5ccd2c3689e338ed1c
Merge "Adding documentation for proxy domain usage"