(PE-4769) Modify the gatling agent run to simulate multiple agents

This commit adds the Puppet Enterprise gatling load test simulation with
a feeder functionality. This means that when the test is run, not only
does it simulate load of X number of nodes, but it will add that number
of nodes into the console for UI testing.

This pull request is also split up into two commits to show the changes required to modify a given gatling scenario into one that has "feeder" functionality.