Don't presume the existence of default dhparam

The default dhparam at /etc/nginx/dhparam/dhparam.pem won't be auto generated with the separate containers setup.

nginx.tmpl

@@ -42,7 +42,9 @@ map $http_upgrade $proxy_connection {
 server_names_hash_bucket_size 128;
 
 # Default dhparam
+{{ if (exists "/etc/nginx/dhparam/dhparam.pem") }}
 ssl_dhparam /etc/nginx/dhparam/dhparam.pem;
+{{ end }}
 
 # Set appropriate X-Forwarded-Ssl header
 map $scheme $proxy_x_forwarded_ssl {
@@ -319,4 +321,4 @@ server {
 {{ end }}
 
 {{ end }}
-{{ end }}
+{{ end }}