Consume notif. from multiple message bus

This patch allow to consume notification from multiple
message bus servers at the same times with configuration like:

 [DEFAULT]
 transport_url = rabbit://rabbit-ceilo:5772/

 [notification]
 messaging_urls = rabbit://rabbit-nova1:5772,rabbit-nova2:5772/
 messaging_urls = rabbit://rabbit-cinder:5772/
 messaging_urls = rabbit://rabbit-neutron:5772/

Implements blueprint multiple-rabbitmq

Co-Authored-By: Ala Rezmerita <ala.rezmerita@cloudwatt.com>

Change-Id: If3bf6f080ce15f2403c3516b433953b0bdf6ea40