Fix TELEGRAF_OPTS expansion in systemd service unit (#3354)

(cherry picked from commit 02baa69)

Author: clheikes

scripts/telegraf.service

@@ -6,7 +6,7 @@ After=network.target
 [Service]
 EnvironmentFile=-/etc/default/telegraf
 User=telegraf
-ExecStart=/usr/bin/telegraf -config /etc/telegraf/telegraf.conf -config-directory /etc/telegraf/telegraf.d ${TELEGRAF_OPTS}
+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
 RestartForceExitStatus=SIGPIPE