roles/nginx: Enable OCSP stapling

Reduces round trip time for clients. Note: I am using a certificate
chain in the `ssl_certificate' directive, so as I understand it, I
don't need to use an explicit trusted intermediate + root CA cert
with the `ssl_trusted_certificate' option. See the nginx docs for
more[0]. Addresses GitHub Issue #5.

Seems to be working, test with:

    $ openssl s_client -connect mjanja.ch:443 -servername mjanja.ch -tls1 -tlsextdebug -status

Look for "OCSP Response" with "Cert Status: good".

[0] http://nginx.org/en/docs/http/ngx_http_ssl_module.html#ssl_stapling

Signed-off-by: Alan Orth <alan.orth@gmail.com>