Add batch_size option to cloudwatch output plugin

Add batch_size option to cloudwatch output plugin, defaulting to 20 datapoints at a time. This mirrors the current limit of maximum of 20 datapoints per call on CloudWatch: http://docs.aws.amazon.com/AmazonCloudWatch/latest/DeveloperGuide/cloudwatch_limits.html

Currently the cloudwatch plugin is majorly broken because of this - uploads will succeed only if metrics are arriving at a rate where there will never be more than 20 datapoints in a single batch. Setting `queue_size` to 20 does not solve the issue as that only changes how often the batch processing is triggered - batch processing will always send all events in queue.

This all would probably require a lot more work to be a good quality plugin, but atleast this fixes the obvious problem that currently exists.