Add Check RabbitMQ Cluster script

Hello James,

I added the check_rabbitmq_cluster script. It checks how many node are in the cluster and throw an warning or a critical alert if the node number are bellow the threshold.

Partition are critical, but sometimes we have many nodes and wanted an critical alerte only if we have an critical number of nodes. So I've just writed quickly this scipt to do that. But I'm wondering if it can do this kind of check into the check_rabbitmq_partition or leave is as a stand alone script.

What do you think?

PS: I don't create the nrpe config for this script.