Certificates: mention make-ssl-cert as easier way to generate self-signed cert for development/test purposes

[Ashley Hooper]

https://help.ubuntu.com/lts/serverguide/certificates-and-security.html

By installing the ssl-cert package, and then running:

$ sudo /usr/sbin/make-ssl-cert generate-default-snakeoil

...a user can be up and running with a 'throwaway' SSL cert that is good enough for dev/test purposes. The Nginx default site configuration already mentions this, and includes the only config required to make this work:

        include snippets/snakeoil.conf;