Add symlink to secrets

More flexibility and less dots in config files.

Implements #65 (closed).

morph/grid/production/grid.nix

@@ -6,7 +6,7 @@ import ../../lib/make-grid.nix {
   config = ./config.json;
   nodes = cfg:
     let
-      sshUsers = import ./secrets/production-users.nix;
+      sshUsers = import ./users.nix;
     in {
     # Here are the hosts that are in this morph network.  This is sort of like
     # a server manifest.  We try to keep as many of the specific details as