roles/nginx: Use snakeoil cert from ssl-cert

Instead of manually creating our own self-signed certificate we can
use the one created automatically by the ssl-cert package on Debian.
This is only used by the dummy default HTTPS vhost.

roles/nginx/tasks/vhosts.yml

@@ -7,11 +7,6 @@
     notify:
       - reload nginx
 
-  - name: Generate self-signed TLS cert
-    command: openssl req -x509 -nodes -sha256 -days 365 -subj "/C=SO/ST=SO/L=snakeoil/O=snakeoil/CN=snakeoil" -newkey rsa:2048 -keyout /etc/ssl/private/nginx-snakeoil.key -out /etc/ssl/certs/nginx-snakeoil.crt -extensions v3_ca creates=/etc/ssl/certs/nginx-snakeoil.crt
-    notify:
-      - reload nginx
-
   - name: Download 4096-bit RFC 7919 dhparams
     get_url:
       url: https://raw.githubusercontent.com/internetstandards/dhe_groups/master/ffdhe4096.pem