Adding ability to graph multiple SQS queues via symlink

- Adding ability to use ":" separator for symlinks. This allows one graph to contain multiple message queues without setting an environment variable. Ex. "ln -s aws_sqs_queue_length_testqueue1:testqueue2:testqueue3 => /usr/share/munin/plugins/aws_sqs_queue_length_"
  - I added this ability because I was unable to specify multiple queues via a symlink using "," as a separator. Munin gave me an error of ```# ERROR: Invalid plugin name 'aws_sqs_queue_length_testqueue1,testqueue2.``` 
- Retaining ability to use the "," separator for queues to graph specified via "os.environ['SQS_QUEUES']"