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

nixos/modules/services/databases/opentsdb.nix

···
 let
   cfg = config.services.opentsdb;
 
-  configFile = pkgs.writeText "opentsdb.conf" ''
-    tsd.core.auto_create_metrics = true
-    tsd.http.request.enable_chunked  = true
-  '';
+  configFile = pkgs.writeText "opentsdb.conf" cfg.config;
 
 in {
 
···
         default = 4242;
         description = ''
           Which port OpenTSDB listens on.
+        '';
+      };
+
+      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
         '';
       };