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/main.yml

@@ -16,8 +16,14 @@
     add_nginx_apt_key is changed or
     add_nginx_apt_repository is changed
 
-- name: Install nginx
-  apt: pkg=nginx cache_valid_time=3600 state=present
+- name: Set nginx packages
+  set_fact:
+    nginx_packages:
+      - nginx
+      - ssl-cert # for ssl-cert-snakeoil.pem in nginx
+
+- name: Install nginx packages
+  apt: pkg={{ nginx_packages }} cache_valid_time=3600 state=present
   tags: nginx, packages
 
 - name: Copy nginx.conf