Add \OC\Preferences::getUsersForKey(Sapp, $key)

A new method to get users according to a key.

This method return an associative array such as array($userid => $configvalue)

Method is unit tested

see https://github.com/owncloud/core/issues/12078
Can you give an example output ? i.e. `array Example: array('UserId' => 'value');`