nixos: Fix priorities of initialHashedPassword.

Regression introduced in f496c3cbe4.

Previously when we used security.initialRootPassword, the default
priority for this option was 1001, because it was a default value set by
the option itself.

With the mentioned commit, it is no longer an option default but a
mkDefault, which is priority 1000.

I'm setting this to 150 now, as test-instrumentation.nix is using this
for overriding other options and because I think it still makes it
possible to simple-override it, because if no priority is given, we get
priority 100.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>

nixos/modules/virtualisation/docker-image.nix

@@ -56,7 +56,7 @@ in {
   services.openssh.startWhenNeeded = false;
 
   # Allow the user to login as root without password.
-  users.extraUsers.root.initialHashedPassword = mkDefault "";
+  users.extraUsers.root.initialHashedPassword = mkOverride 150 "";
 
   # Some more help text.
   services.mingetty.helpLine =