'Exchange' names should be configurable

[Steve McLellan]

Each plugin sets an 'exchange' name in code currently. Exchanges are configurable per service; we need instead to have a configuration section mapping service names to exchanges and topics. For instance:

    [service_notifications]
    nova=nova,searchlight_indexer
    neutron=neutron,searchlight_indexer
    cinder=openstack,searchlight_indexer

I would also allow the following, and default the topic to the notifications_topic setting we currently have:

    nova=nova

[Steve McLellan (sjmc7)]

notifications_topics_exchanges is an existing configuration setting, so this isn't perhaps so urgent. It would still be nicer to set up notifications for each service once rather than in each plugin configuration section.

[Travis Tripp (travis-tripp)]

Lei, Are you still working on this?

[Lei Zhang (lei-a-zhang)]

Hi, Travis, sorry for the delay, have been occupied these days, I did some investigation before, but probably cannot start it immediately, I'll mark it unoccupied, and come back to work on it if no one takes the bug.