Set default auth_url from django settings when auth_url is None
Set default auth_url from django settings when auth_url is None

This is useful when users(client web forms) just provide
username/password to authenticate with django_openstack_auth. In this
case, they can still login if keystone version == 2.0 since keystone
v2.0 only requires username/password and auth_url to authenticate. In
most cases, auth_url can get from django(horizon)'s settings.

Fix-bug: #1316490
Change-Id: I2ed24238adb79b6ef33e4bf20232b6a924ad0b1f

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