roles/nginx: Only check WordPress variables is vhost is using WordPress

This variable is used to control the FastCGI cache, and doesn't
need to be checked if the vhost isn't using WordPress.

roles/nginx/templates/vhost.conf.j2

@@ -91,6 +91,7 @@ server {
     include extra-security.conf;
 }
 
+{% if has_wordpress == True %}
 # Check if a user is logged in
 # if so, set $wordpress_logged_in = 1
 # otherwise, set $wordpress_logged_in = 0
@@ -102,3 +103,4 @@ map $http_cookie $wordpress_logged_in {
 
     ~wordpress_logged_in 1;
 }
+{% endif %}