Add multiple rabbit hosts support

* Use new oslo.messaging conf options for rabbit_hosts and
  rabbit_ha_queues for glance::notify::rabbitmq class
* Warn on deprecated rabbit_durable_queues and use amqp_* instead
* Fix/add rspecs
* Fix a mess with wrong notification_driver usage
* Update README with usage example

DocImpact
Closes-bug: #1259193
Closes-bug: #1312054
Closes-bug: #1312658
Closes-bug: #1313553

Change-Id: I16b8e71db20602986a7afe8d24e7faba91a0997f
Signed-off-by: Bogdan Dobrelya <bdobrelia@mirantis.com>