Set use_letsencrypt to true for nginx role

The variable name is misleading as this really does is install the
certbot client and its dependencies, and we generally want this to
always happen. If a host doesn't want it, they can override it in
their host vars.

Perhaps I should rename this variable to "bootstrap_letsencrypt" or
something so it is more accurate.

roles/nginx/defaults/main.yml

@@ -16,6 +16,10 @@ nginx_ssl_buffer_size: 1400
 nginx_ssl_dhparam: /etc/ssl/certs/dhparam.pem
 nginx_ssl_protocols: 'TLSv1 TLSv1.1 TLSv1.2'
 
+# install certbot + dependencies?
+# True unless you're in development and using "localhost" + snakeoil certs
+use_letsencrypt: True
+
 # Directory root for Let's Encrypt certs
 letsencrypt_root: /etc/letsencrypt/live