dd-agent: Add default config files of dd-agent and auto_conf dir

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)

pkgs/tools/networking/dd-agent/default.nix

@@ -64,6 +64,9 @@ in stdenv.mkDerivation rec {
     ln -s $out/agent/dogstatsd.py $out/bin/dogstatsd
     ln -s $out/agent/ddagent.py $out/bin/dd-forwarder
 
+    # Move out default conf.d so that /etc/dd-agent/conf.d is used
+    mv $out/agent/conf.d $out/agent/conf.d-system
+
     cat > $out/bin/dd-jmxfetch <<EOF
     #!/usr/bin/env bash
     exec ${python}/bin/python $out/agent/jmxfetch.py $@