Set DJANGO_SETTINGS_MODULE envvar in doc/source/conf.py

This fixes the issue that the doc is not generated properly
in gate-django_openstack_auth-docs job.

Change-Id: I5780ee32856a9b1fd4aed5b6a1a84d319aaf464e
Closes-Bug: #1369064
Merge "Set DJANGO_SETTINGS_MODULE envvar in doc/source/conf.py"