roles/nginx: Use variable for nginx_ssl_dhparam

I went years without realizing that I was hard coding the file dest
in this particular task.

roles/nginx/tasks/vhosts.yml

@@ -16,7 +16,7 @@
     get_url:
       url: https://raw.githubusercontent.com/internetstandards/dhe_groups/master/ffdhe4096.pem
       checksum: sha256:64852d6890ff9e62eecd1ee89c72af9af244dfef5b853bcedea3dfd7aade22b3
-      dest: /etc/ssl/certs/dhparam.pem
+      dest: "{{ nginx_ssl_dhparam }}"
     notify:
       - reload nginx