Create specific SSF rack configuration

Allow xeon machines to optionally inherit SSF configuration such as the
NFS mount point and the network configuration.

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

m/common/ssf/ssh.nix

@@ -0,0 +1,8 @@
+{
+  # Connect to intranet git hosts via proxy
+  programs.ssh.extraConfig = ''
+    # Connect to BSC machines via hut proxy too
+    Host amdlogin1.bsc.es armlogin1.bsc.es hualogin1.bsc.es glogin1.bsc.es glogin2.bsc.es fpgalogin1.bsc.es
+      ProxyCommand nc -X connect -x hut:23080 %h %p
+  '';
+}