Self-signed key and certificate for bootstrapping.

There's a chicken and egg issue with new servers, Nginx and Let's
Encrypt. The Nginx setup expects an SSL key and certificate and DH
params file. But the Let's Encrypt challenge has to have Nginx running.
So as a bootstrap step, create an SSL key and self-signed certificate.
Nginx will start (although clients over HTTPS will complain, the Let's
Encrypt challenge requires just HTTP) and then I'll be able to pass the
Let's Encrypt challenge.