Avoid repeated variable update

Make a break after first assignment to skip unwanted variable update.

Change-Id: Ie376cdd6845b6fbbe479b691fd921a12259005f1
diff --git a/nginx/server.sls b/nginx/server.sls
index f8c9305..222f06c 100644
--- a/nginx/server.sls
+++ b/nginx/server.sls
@@ -82,6 +82,7 @@
 {%- for site_name, site in server.get('site', {}).iteritems() %}
 {%- if site.get('ssl', {}).get('enabled') and site.ssl.get('mode', 'secure') == 'secure' %}
   {%- do generate_dhparams.update({ 'enabled': True }) %}
+  {%- break %}
 {%- endif %}
 {%- endfor %}