Fix EZP-22915: do not use eZINI when checking path in eZDir::recursiveDelete()

This avoid depending on global ini cache when trying to clear it.
The config parameter needs to be known before eZINI is loaded, so it is
defines in config.php

https://jira.ez.no/browse/EZP-22915

See PR #1006 :
49583e1 introduced an "egg or chicken" dilemma : when you use ezcache.php to clear the global ini cache, eZDir::recursiveDelete() will read the old settings from the cache, and may disallow you to clear the cache if your old settings disallow it.
Nitpick:
This doesn't seem to allow for multiple directories, should it?
Also, IMHO, the empty default `''` doesn't make much sense, ideally it should be self-explanatory.