Add a flag for concurrency in the worker service script

[awest1339]

• Celery lets you specify the # of processes that should be created on a single worker instance
• I've found some increase in productivity by bumping the number of concurrent processes to equal the # of available sandbox VM's in Cuckoo.
• Celery docs here: http://docs.celeryproject.org/en/latest/userguide/workers.html#concurrency