Make keystone notification topics configurable

Make keystone notification topics configurable

By default keystone sends notifications to the 'notifications'
topic. This is consumed by default by ceilometer. However, when
ceilometer is not enabled, rabbitmq accumulates these notifications
since nothing is consuming them. This results in the queue consuming
rabbitmq's memory.

With this capability, we can now dynamically configure notification
topics depending on the 'keystone_notification_topics' hiera key.

Related-Bug: #1729293
Change-Id: I4dcce73446633c08ea37ba567610eec398094036
(cherry picked from commit bbe7d9effe51324ef5fa75e40f9d0ca74544d484)