config: rename .ini settings to avoid beaker-session deprecation warnings

Gets rid of:

data/env/lib/python2.7/site-packages/tg/wsgiapp.py:68
data/env/lib/python2.7/site-packages/tg/wsgiapp.py:68: DeprecationWarning: Session options should start with session. instead of baker.session.
app_wrapper = wrapper(self.wrapped_dispatch, self.config)

Old .ini files will still work, but one day upstream will drop the deprecated functionality ...