opentsdb nixos module: Add option for defining OpenTSDB's configuration

nixos/modules/services/databases/opentsdb.nix

@@ -5,10 +5,7 @@ with lib;
 let
   cfg = config.services.opentsdb;
 
-  configFile = pkgs.writeText "opentsdb.conf" ''
+  configFile = pkgs.writeText "opentsdb.conf" cfg.config;
-    tsd.core.auto_create_metrics = true
-    tsd.http.request.enable_chunked  = true
-  '';
 
 in {
 
@@ -59,6 +56,17 @@ in {
         '';
       };
 
+      config = mkOption {
+        type = types.lines;
+        default = ''
+          tsd.core.auto_create_metrics = true
+          tsd.http.request.enable_chunked  = true
+        '';
+        description = ''
+          The contents of OpenTSDB's configuration file
+        '';
+      };
+
     };
 
   };