nginx/site: Disable certificate check when we have multiple backends

Nginx requires that all backend certificates need to match name defined
in ProxyPass directive:

https://trac.nginx.org/nginx/ticket/1307

roles/nginx/site/templates/site.conf.j2

@@ -27,6 +27,8 @@ server {
 {% elif proxy is defined %}
     location / {
 {%  if proxy is not string %}
+        # https://trac.nginx.org/nginx/ticket/1307
+        proxy_ssl_verify off;
         proxy_pass https://upstream_{{ site }};
 {%  else %}
         proxy_pass {{ proxy }};