RPC and fault tolerant configurations for RabbitMQ

Austin release of Nova RPC mappings deals with intermittent network connectivity only. In order to support RabbitMQ clusters and active/passive brokers, more advanced Nova RPC mappings need to be provided, such as strategies to deal with failures of nodes holding queues within clusters and/or master/slave failover for active/passive replication.

Currently, the message queue configuration variables are tied to RabbitMQ from nova/flags.py. In particular, only one rabbitmq host is provided and it is assumed, for simplicity of the deployment, that a single instance is up and running. In face of failures of the RabbitMQ host (e.g. disk or power related), Nova components cannot send/receive messages from the queueing system until it recovers. To provide higher resiliency, RabbitMQ can be made to work in an active/passive setup, such that persistent messages that have been written to disk on the active node are able to be recovered by the passive node should the active node fail. If high-availability is required, active/passive HA can be achieved by using shared disk storage, heartbeat/pacemaker, and possibly a TCP load-balancer in front of the service replicas. Although this solution ensures higher level of transparency to the client-side such as Nova API, Scheduler, and Compute (e.g. no or minimal fail-over strategies are required in the Nova RPC mappings) it still represents a bottleneck of the overall architecture, it may require expensive hardware to run, and hence it is far from ideal.