rt and RabbitMQ do not need to run if the collector will not run

[mikeiannacone]

If the collectors are set to "once" mode, rt and RabbitMQ do not need to run after the initial collecting has completed.

If the collectors are set to "once", they will simply not be added to the supervisor configuration. However, this will not work here, because rabbitmq is not managed that way, and because supervising rt during the initial loading is still desirable.

We need to decide on the best way to pass/store/use this option. (eg, the collector role could read the stucco_collector_mode value from its config, set an environment variable, and then cleanup.sh or similar could check that env. variable. But I'd like to find a simpler way.)

Also note if rt is not configured to run after the initial processing, the VMs memory can potentially be lowered after the loading finishes.

[mikeiannacone]

This is a very minor issue, but still open. Probably not a great 'first issue' for someone, because it involves changes to several different components.