Log to systemd journal

Let's align to InfluxDB 1.0 logging policy and log to systemd journal by
default.

closes #1732

scripts/telegraf.service

@@ -6,9 +6,7 @@ After=network.target
 [Service]
 EnvironmentFile=-/etc/default/telegraf
 User=telegraf
-Environment='STDOUT=/var/log/telegraf/telegraf.log'
-Environment='STDERR=/var/log/telegraf/telegraf.log'
-ExecStart=/bin/sh -c "exec /usr/bin/telegraf -config /etc/telegraf/telegraf.conf -config-directory /etc/telegraf/telegraf.d ${TELEGRAF_OPTS} >>${STDOUT} 2>>${STDERR}"
+ExecStart=/usr/bin/telegraf -config /etc/telegraf/telegraf.conf -config-directory /etc/telegraf/telegraf.d ${TELEGRAF_OPTS}
 ExecReload=/bin/kill -HUP $MAINPID
 Restart=on-failure
 KillMode=control-group