Rework Gatling tests

[kciesielski]

Gatling version 3.11 introduces changes which no longer allow us to run simulations programmatically from our PerfTestSuiteRunner. I removed the runner entirely, together with our custom reporters, leaving latency histograms. Our experience with performance tests shows that, in order to get unbiased results, test server and simulations shouldn't be run in aggregatted way. It is preferred to start a single test server and run simulations on it, so I'm leaving the ServerRunner and some instructions about how to use perfTests/Gatling/testOnly with parameterized user count and duration.