messaging: add cluster_node_type param

Allow the end-user to choose starting RabbitMQ in RAM or on the disk.
Backward compatible since the default value is to 'disk'.

Signed-off-by: Emilien Macchi <emilien.macchi@enovance.com>
Merge pull request #401 from enovance/messaging_node_type

messaging: add cluster_node_type param