Set DJANGO_SETTINGS_MODULE envvar in doc/source/conf.py
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

redhat-openstack/django_openstack_auth,dan1/django_openstack_auth_proto,ashokkumarprajapati/openstack_auth_remote,promptworks/django_openstack_auth,jamielennox/django_openstack_auth,promptworks/django_openstack_auth,dan1/django_openstack_auth_proto,redhat-openstack/django_openstack_auth,ging/django_openstack_auth,ging/django_openstack_auth