to /etc/dd-agent/conf.d by default, and make sure
/etc/dd-agent/conf.d is used.
Before NixOS 17.03, we were using dd-agent 5.5.X which
used configuration from /etc/dd-agent/conf.d
In NixOS 17.03 the default conf.d location is first used relative,
meaning that $out/agent/conf.d was used without NixOS overrides.
This change implements similar functionality as PR #25288, without
breaking backwards compatibility.
(cherry picked from commit 77c85b0ecbc1070d7adff31b339bede92e4193fa)
