lnd: only set tor.active on enforceTor

This also enables the test scenario 'netnsRegtest' introduced in a
later commit by fixing the following bug:
For unknown reasons, when tor.active=true and tor is not running, lnd
fails with a tor connection error on netns-isolation, but runs fine
without netns-isolation.

modules/lnd.nix

@@ -25,7 +25,7 @@ let
     bitcoin.active=1
     bitcoin.node=bitcoind
 
-    tor.active=true
+    ${optionalString (cfg.enforceTor) "tor.active=true"}
     ${optionalString (cfg.tor-socks != null) "tor.socks=${cfg.tor-socks}"}
 
     bitcoind.rpchost=${bitcoindRpcAddress}:${toString bitcoind.rpc.port}