Add support for Logstash service in system benchmarking

With the Logstash service added in #1458, system benchmarking should also support using the logstash-output in the benchmarking agent policies.

Like done for [system testing](https://github.com/elastic/elastic-package/blob/main/internal/testrunner/runners/system/runner.go#L559), if `stack.logstash_enabled: true` is set in the current profile's config use the logstash output over the default.