[systemd] apply sysconfig/default settings on launch too. (#3489)

Author: truthbk

packaging/datadog-agent.service

@@ -5,6 +5,8 @@ After=network.target
 [Service]
 Type=forking
 User=dd-agent
+EnvironmentFile=-/etc/default/datadog-agent
+EnvironmentFile=-/etc/sysconfig/datadog-agent
 ExecStart=/opt/datadog-agent/bin/start_agent.sh
 ExecStop=/opt/datadog-agent/bin/supervisorctl -c /etc/dd-agent/supervisor.conf shutdown
 

packaging/start_agent.sh

@@ -5,4 +5,9 @@
 
 PATH=/opt/datadog-agent/embedded/bin:/opt/datadog-agent/bin:$PATH
 
+if [ "$DATADOG_ENABLED" = "no" ]; then
+    echo "Disabled via DATADOG_ENABLED env var. Exiting."
+    exit 0
+fi
+
 exec /opt/datadog-agent/bin/supervisord -c /etc/dd-agent/supervisor.conf