Fix redirect to actual index view rather than '/' on logout for DBAuthView.
Fix redirect to actual index view rather than '/' on logout for DBAuthView.

The other AuthVies use self.appbuilder.get_url_for_index. Make sure this
one does too.

dpgaspar/Flask-AppBuilder,dpgaspar/Flask-AppBuilder,dpgaspar/Flask-AppBuilder,dpgaspar/Flask-AppBuilder