Support notification Transports per oslo messaging notifications

Large clouds where the cloud is being maintained with multiple regions,
operators usually configure global components(for ex: Designate, Ceilometer)
in the DR(Disaster Recovery) fashion. The core services like Nova and
Neutron are then configured to send oslo.messaging notifications to both
the region based components(for ex: Searchlight) and at the same time to
global components. Currently oslo.messaging only allows to send all the
notifications to just one underlying messaging transport(for ex:
one rabbitmq cluster which has deployed region based components).
This spec proposes to add feature into oslo.messaging which enables operators
to define different transport_url for each notification so that they can be
sent to the different messaging transport(for ex: rabbitmq clusters)