Add hut SSH configuration from outside SSF LAN

Reviewed-by: Aleix Boné <abonerib@bsc.es>

m/module/ssh-hut-extern.nix

@@ -0,0 +1,9 @@
+{
+  programs.ssh.extraConfig = ''
+    Host ssfhead
+      HostName ssflogin.bsc.es
+    Host hut
+      ProxyJump ssfhead
+      HostName xeon07
+  '';
+}