SLING-3154 Add Topology Message Verification to the Discovery service.

Adds signatures and optional encryption using JDK provided HmacSHA256 and 
AES 128 bit encryption to discovery messages. To enable the Config component
must be configured. Once configured, the IP whitelist is disabled and replaced 
by message validation.

git-svn-id: https://svn.apache.org/repos/asf/sling/trunk@1530702 13f79535-47bb-0310-9956-ffa450edef68