roles/nginx: Set nginx SSL buffer size to 1400

istlsfastyet.com recommends setting the buffer size to 1400 so it
can fit into a single MTU.  nginx default is 16k!

http://nginx.org/en/docs/http/ngx_http_ssl_module.html#ssl_buffer_size

Signed-off-by: Alan Orth <alan.orth@gmail.com>
roles/nginx: Format and add comments to nginx https config

Signed-off-by: Alan Orth <alan.orth@gmail.com>
roles/nginx: Set nginx SSL session timeout to 24 hours

Default is 5 minutes, but it seems like unless you're a high-traff-
ic site, there's no need to expire sessions so quickly.  Also, the
istlsfastyet.com configs are using 24 hours, so surely we can.

Signed-off-by: Alan Orth <alan.orth@gmail.com>
roles/nginx: Disable SSL session tickets

Session tickets increase performance, but decrease security, so
let's just turn them off.  See the following posts:

- https://timtaubert.de/blog/2014/11/the-sad-state-of-server-side-tls-session-resumption-implementations/
- https://www.imperialviolet.org/2013/06/27/botchingpfs.html
- https://github.com/igrigorik/istlsfastyet.com/blob/master/nginx/includes/ssl.conf

Signed-off-by: Alan Orth <alan.orth@gmail.com>
roles/nginx: Enable SPDY header compression

Recommended by Ilya Grigorik to be set to 6.

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