[systemd] apply sysconfig settings on launch too.

[truthbk]

What does this PR do?

Honor sysconfig settings when launching with systemd service.

Motivation

Not working in the mentioned scenario: #3301

[olivielpeau]

$AGENTSYSCONFIG doesn't exist here, I'll update the PR

[olivielpeau]

@truthbk This seems to work well but the way systemd logs what our script echoes is a bit weird when DATADOG_ENABLED is set to no:

This is what systemd logs once service datadog-agent start is called on an ubuntu 16.04 VM. The agent does not actually start (as expected) but the logs might be misleading:

$ journalctl -u datadog-agent
Aug 23 18:46:29 ubuntu-xenial systemd[1]: Starting "Datadog Agent"...
Aug 23 18:46:29 ubuntu-xenial supervisorctl[28008]: ERROR: unix:///opt/datadog-agent/run/datadog-supervisor.sock no such file (already shut down?)
Aug 23 18:46:29 ubuntu-xenial systemd[1]: Started "Datadog Agent".

whereas /var/log/syslog does contain what we echo from start_agent.sh:

Aug 23 18:46:29 ubuntu-xenial systemd[1]: Starting "Datadog Agent"...
Aug 23 18:46:29 ubuntu-xenial start_agent.sh[28007]: Disabled via DATADOG_ENABLED env var. Exiting.
Aug 23 18:46:29 ubuntu-xenial supervisorctl[28008]: ERROR: unix:///opt/datadog-agent/run/datadog-supervisor.sock no such file (already shut down?)
Aug 23 18:46:29 ubuntu-xenial systemd[1]: Started "Datadog Agent".

I think this is related to the forking type of our service, which might make systemd only log what the forked processes echo to stdout.

Not sure this is a blocker, I'm concerned this may make things harder to understand and debug by users. Let me know what you think though, and feel free to merge if you think this is OK.

[truthbk]

The error: ERROR: unix:///opt/datadog-agent/run/datadog-supervisor.sock no such file (already shut down?) we see is due to ExecStop getting called. I think that since the script is exiting cleanly, the service is considered to have been up. Then systemd detects the process is gone, and calls ExecStop.

[olivielpeau]

Going to go ahead and merge this, we can fix the slightly misleading log entry later.