Add support for RabbitMQ Mirrored Queues.

Previously, the openstack::quantum class could only have
a single rabbit broker by using the rabbit_host parameter.

This patch allows users to specify an array of rabbitmq brokers by
introducing the rabbit_hosts parameter to leverage
Rabbit Mirrored Queues (rabbit_ha_queues).

Defaults to false for backwards compatability and to not
implement rabbit_ha_queues.

Change-Id: I24f8dadde3324e589ae38603d85acd61ccf4d911
Merge "Add support for RabbitMQ Mirrored Queues."