Restore "redirect to login page after logout" behaviour

The expected behaviour is to redirect to the login page after a user
gets logged out. This was changed accidentally during a refactoring.

This will get rid of the following error 500 in Horizon when logging
out: TemplateDoesNotExist: registration/logged_out.html.

Change-Id: Id23666e84fce1e3a212b066fcdaf71a17e4898d9
Closes-Bug: #1332167
Merge "Restore "redirect to login page after logout" behaviour"