Adds support for RabbitMQ Clustering

Prior to this change, only a single RabbitMQ Broker
could be deployed by the nova::rabbitmq class.

This change implements the cluster_disk_nodes parameter
that can enable RabbitMQ clustering.

Defaults to false for backwards compatibility.

Change-Id: Iac2d04e855adb53fbb9fe2a23f8773a692162744
Add missing pre_condition to nova::db::mysql tests

mysql::server was missing from pre_condition causing tests to fail.

Change-Id: I2cb93be62eac9fe8d71c8fd575780a13b0b27fd2
Merge "Adds support for RabbitMQ Clustering"