nixos/tests/letsencrypt: Fix nginx options

The enableSSL option has been deprecated in
a912a6a2

, so we switch to using onlySSL.

I've also explicitly disabled enableACME, because this is the default
and we don't actually want to have ACME enabled for a host which runs an
actual ACME server.

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