Set default auth_url from django settings when auth_url is None.

[xiaohanyu]

By default, users should pass username/password/auth_url in order to get authenticated from django_openstack_auth.

However, by default, in general we can often get auth_url from django's conf.settings.

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.

This makes it more consistent with django.contrib.auth.

[Julie Pichon (jpichon)]

Patch at https://review.openstack.org/#/c/89323/

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/89323
Committed: https://git.openstack.org/cgit/openstack/django_openstack_auth/commit/?id=e239fb78e07a86daee3821ccabc3e929cf5ab79f
Submitter: Jenkins
Branch: master

commit e239fb78e07a86daee3821ccabc3e929cf5ab79f
Author: Xiao Hanyu <email address hidden>
Date: Mon Apr 21 17:21:31 2014 +0800

    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