nixos: Fix priorities of initialHashedPassword.

Regression introduced in f496c3cbe4a6a2db88892d14609618e449744ed6.

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>