Picking up environment vars from file for trace-agent

[runlevel5]

What does this PR do?

Let trace-agent pick up common DD_* environment variables from /etc/datadog-agent/environment

Motivation

So that we have consistent way to configure daemons.

Possible Drawbacks / Trade-offs

Not that I am aware of. Unless the core team has different intention for this feature.

[bits-bot]

All committers have signed the CLA.

[spencergilbert]

Thanks for the contribution @runlevel5 I don't see why we wouldn't include this here - and also why we shouldn't do so for the rest of the agent services. I'm going to check internally that there's no issues updating the other files. Assuming there isn't would you like to update those files as well, or should I commit those changes?

Additionally the failed CircleCI integration tests should pass when rebased, there was an issue with the docker-compose version that was being used - I believe our fix is on main at this point.