FOR REVIEW ONLY: Use gatling's `group` feature to organize agent requests

This commit shows how we could modify the test runner to group each node into it's own `group` of requests in Gatling.  I'm not entirely sure what all of the pros and cons are, but I am starting to think that for multi-node scenarios, this will be necessary in order for our post-processing plugin to be able to calculate an accurate mean agent response time.

For now this pull request is just for discussion purposes.