roles/nginx: Allow usage of Let's Encrypt certs

Hosts can specify use_letsencrypt: 'yes' in their host_vars. For
now this assumes that the certificates already exist (ie, you have
to manually run Let's Encrypt first to register/create the certs).

roles/nginx/defaults/main.yml

@@ -16,8 +16,8 @@ nginx_ssl_buffer_size: 1400
 nginx_ssl_dhparam: /etc/ssl/certs/dhparam.pem
 nginx_ssl_protocols: 'TLSv1 TLSv1.1 TLSv1.2'
 
-# TLS key directory
-tls_key_dir: /etc/letsencrypt/live
+# Directory root for Let's Encrypt certs
+letsencrypt_root: /etc/letsencrypt/live
 
 # stable is 1.10.x
 # mainline is 1.11.x