Ignore output from cron execution (#26)

Some processing in the cron task occasionaly produce output that result
in an e-mail being sent.

On our fleet, it happens less than 1 time a day, and only contain 1 or
more of the following line:

```
libpng warning: iCCP: known incorrect sRGB profile
```

These warning are not actionable, so we can probably just ignore them.

Author: neomilium

manifests/base.pp

@@ -27,7 +27,7 @@
   }
 
   cron { 'nextcloud-cron':
-    command => "/usr/bin/php -f ${nextcloud::current_version_dir}/cron.php",
+    command => "/usr/bin/php -f ${nextcloud::current_version_dir}/cron.php > /dev/null 2>&1",
     user    => $nextcloud::user,
     minute  => '*/15',
   }