Ensure default addon paths are valid

The tarball packaging moves all modules from /addons to / openerp/addons and removes the /addons directory. Because default addons paths are not checked before being added, downloading, unpacking and running a tarball without explicitly specifying an addons path will blow up when `http.py` tries to list & load modules as it feeds an invalid path to `os.listdir`.

Fixes the issue by *not* adding invalid default paths to the addons_path in the first place.

/cc @KangOl @odony 
I think that logging the non-existence of a path could be useful, it needs to be corrected and a warning may help to find the misconfiguration.