Make family file generator work with broken user_config

This reverts the removal of os.path.exists monkey patching that was
done in r10886 (git sha1 971d1150f66ddde5ea53def0c7e8f0e1edbd763c),
which means config.py will not find any user-config.py file. However,
we also restore the functionality after importing config, because
importing wikipedia requires a working os.path.exists.

Change-Id: I050d050dee536d0c44589651bff4dce866c50942
Merge "Make family file generator work with broken user_config"