[FEAT] config2: Allow to test whether a path would work

The config2 now also allows to "simulate" that a specific base directory
does exist, to get rid of the warning when the user doesn't has to do
anything to make it find the user-config.py.

Change-Id: I88691ea634f0579aa28ee88d852d9073283c4ae7
Merge "[FEAT] config2: Allow to test whether a path would work"