Fix errors after install

Fix errors after install

When installing ownCloud the first time the first thing that an admin saw was an error message:

> Error   PHP   touch(): Unable to create file owncloud.log because Permission denied at /var/www/owncloud/lib/private/log/owncloud.php#48

Or something related. This lead to a lot confusion as can be seen in our forum and in our issue tracker. This change should make the error messages disappear in most cases (e.g. where the file can actually be written). To test this:

1. On master install ownCloud and check owncloud.log => Error message
2. On this branch install ownCloud and check owncloud.log => No error message

Fixes https://github.com/owncloud/core/issues/13736 and https://github.com/owncloud/core/issues/12893