Suppress warning in setup if config.php doesn't exist

More of a cosmetic issue …

Delete config/config.php (and data-dir) to reset your instance, and let php-warnings show and you'll get:
```
Warning: include(/testcloud/config/config.php): failed to open stream: No such file or directory in /testcloud/lib/config.php on line 147

Warning: include(): Failed opening '/testcloud/config/config.php' for inclusion (include_path='/testcloud/lib:.:') in /home/http/testcloud/lib/config.php on line 147
```

@DeepDiver1975 @icewind1991 @Raydiation @karlitschek