[BUGFIX] fileadmin is hardcoded in install tool

In the class "DefaultFactory", "fileadmin" is hardcoded.
The function "getDefaultStructureDefinition"
must take care of the $GLOBALS['TYPO3_CONF_VARS']
['BE']['fileadminDir'] configuration variable.

Resolves: #53872
Releases: 6.2
Change-Id: I17c836a58ea70d218170a33e28ca578bb50eef0b
Reviewed-on: https://review.typo3.org/25640
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn
Reviewed-by: Stefan Neufeind
Reviewed-by: Xavier Perseguers
Tested-by: Xavier Perseguers
Reviewed-by: Markus Klein
Tested-by: Markus Klein