Set default max_failing_days to 15 days.

data.c

@@ -3060,7 +3060,7 @@ int instance_failure(const char *url, int op)
     case 0: /** check **/
         if ((mt = mtime(fn)) != 0.0) {
             /* grace time */
-            double seconds_failing = xs_number_get(xs_dict_get_def(srv_config, "max_failing_days", "30"))
+            double seconds_failing = xs_number_get(xs_dict_get_def(srv_config, "max_failing_days", "15"))
                 * (24 * 60 * 60);
 
             if ((double)time(NULL) - mt > seconds_failing)