logstash service: make log verbosity configurable.

The log verbosity level is now configurable.

The logstash docs don't mention all of the verbosity flags, but you can view them here: https://github.com/elasticsearch/logstash/commit/9b5a60d561ec014b5ba7d27997ac80f320aeeff5#diff-4fbd59cdf63cce1b9ce69be1aa58c28dR120