The recommended TLS configuration comes with `ssl_stapling on` and
`ssl_stapling_verify on`. However, this last directive also requires
the use of `ssl_trusted_certificate` to verify the received answer.
When using `enableACME` or similar, we can help the user by providing
the correct value for the directive.

The result can be tested with:

    openssl s_client -connect web.example.com:443 -status 2> /dev/null

Without OCSP stapling, we get:

    OCSP response: no response sent

After this change, we get:

    OCSP Response Data:
        OCSP Response Status: successful (0x0)
        Response Type: Basic OCSP Response
        Version: 1 (0x0)
        Responder Id: C = US, O = Let's Encrypt, CN = Let's Encrypt Authority X3
        Produced At: Aug 30 20:46:00 2018 GMT

nixos/nginx: ensure TLS OCSP stapling works out of the box with LE