If devstack is deployed in the VM with defined
public IP address (like 192.168.10.6) it is not possible to
access the Horizon from the browser.
This is because DEBUG=True means that ALLOWED_HOSTS, if not set,
is equal to ['localhost', '127.0.0.1', '[::1]'] according
to Django's documentation.
Change-Id: I74ae99569dafa10eee7066713a05fb49183e3fca