[QA] usecase for storage credentials in config.php needed

Seen in 10.10.0 RC2 and 10.9.1

External storages have an option for Credentials hardcoded in config file - This feature allows
to store passwords in config.php

We can use this by adding an entry in config.php like this:

'ftpcred' => 'ftp123456data',

Or, to distinguish multiple servers, an array syntax is available:

'ftpcred2' => [ 'localhost' => 'ftp123456data' ],

The feature does not allow to store the user name that belongs to the password. While testing, I found that the name of the current owncloud user is used. This means, that each owncloud user must have an account in the storage server, and all users there have to have the same password.
@pmaier1 It would be great to document a scenario, where this option is helpful. Possibly broken by design?

The docs previously had a different syntax, with sub-keys [ password => ...] which does not work with 10.9.1 or 10.10.0 -> https://github.com/owncloud/docs-server/issues/311

Start by reviewing the config.php credential examples in this issue and the earlier docs-server issue #311, then verify the current behavior in an external-storage setup. Done means documenting a valid, useful scenario and the supported syntax, or clearly recording the limitation if the username and password behavior makes the option unsuitable.