Fix weboob data dir path for the self-hosted case

In the current setting, if I go from a clean Kresus install and use npm install -g kresus, it will install it in the /usr/local/lib/node_modules/kresus directory. Most of the time, this directory is not owned by the current user, so you have to be root or use sudo.

Weboob modules are installed by kresus in the $workdir/weboob/data directory. As documented in weboob.py, $workdir is the build directory, so /usr/local/lib/node_modules/kresus/build/server. But if a simple user runs the kresus executable, then they don't have write permissions in this directory! So weboob modules will never work.

@nicofrand, mind to have a look, please?