Fall back to default port for strfry proxy

When we don't override it elsewhere

site-cookbooks/kosmos_strfry/templates/nginx_conf_strfry.erb

@@ -1,6 +1,6 @@
 upstream _strfry {
 <% @upstream_hosts.each do |host| %>
-  server <%= host %>:<%= @upstream_port %>;
+  server <%= host %>:<%= @upstream_port || "7777" %>;
 <% end %>
 }