Update WSGI app creation to be compatible with Django 1.7

This resolves the following error when trying to run Horizon:

AppRegistryNotReady: The translation infrastructure cannot be
initialized before the apps registry is ready. Check that you don't
make non-lazy gettext calls at import time.

and is backwards-compatible with our currently supported versions (back
to 1.4).

Change-Id: I1e59f2bf6633a34cfe9230aa028f345e5fa70f94
Closes-Bug: #1382023
(cherry picked from commit 686af384c201148c3aebcfbf18b2527587d70ea4)