bundles/nginx: ensure we're doing letsencrypt, since we're enforcing ssl

bundles/nginx/items.py

@@ -1,3 +1,8 @@
+# TODO rework this to support specifying a certificate instead of
+# relying on letsencrypt for the specific domain (for example to
+# support wildcard certificates
+assert node.has_bundle('letsencrypt'), 'nginx needs letsencrypt'
+
 files = {
     '/etc/nginx/nginx.conf': {
         'content_type': 'mako',