make secrets dir location configurable

Users of the nix-bitcoin modules shouldn't be forced to add an extra
dir under root.
The secrets location is unchanged for the default node config.

modules/spark-wallet.nix

@@ -8,7 +8,7 @@ let
   dataDir = "/var/lib/spark-wallet/";
   onion-chef-service = (if cfg.onion-service then [ "onion-chef.service" ] else []);
   run-spark-wallet = pkgs.writeScript "run-spark-wallet" ''
-    CMD="${pkgs.nix-bitcoin.spark-wallet}/bin/spark-wallet --ln-path ${cfg.ln-path} -Q -k -c /secrets/spark-wallet-login"
+    CMD="${pkgs.nix-bitcoin.spark-wallet}/bin/spark-wallet --ln-path ${cfg.ln-path} -Q -k -c ${config.nix-bitcoin.secretsDir}/spark-wallet-login"
     ${optionalString cfg.onion-service
       ''
       echo Getting onion hostname