Allow user to choose to use between greenthread or stdlib threads in rabbitmq driver

On rabbitmq driver add the possibility to users, by using config, to choose to use greenthreads ou python stdlib threads.
In production environment we facing many issues related to greenthread from eventlet used in uwsgi. I propose by default to continue to use eventlet threads and for advanced customer to use stdlib thread if needed (example nova-api AMQP heartbeat)