Add support for Logstash service in system benchmarking

[ebeahan]

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, if stack.logstash_enabled: true is set in the current profile's config use the logstash output over the default.

[ebeahan]

Also handing the ingest node stats will be different. Unsure what metrics/stats are exposed in Logstash compared to ES ingest node stats.

[bhapas]

#1615 adds support for logstash in system benchmarking. Logstash does not support metrics similar to ingest node yet. Hence closing this issue.

[bhapas]

Issue created on logstash elastic integration filter plugin to resolve this. elastic/logstash-filter-elastic_integration#119