Configuring Logstash when installed as a service
you got configuration of logstash in directory /etc/logstash/conf.d/
You got all paths in /etc/init.d/logstash
# logstash configuration directory
CONF_DIR=/etc/logstash/conf.d
I got there file /etc/logstash/conf.d/logstash.conf which is automaticlly generated by puppet logstash module :)