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
Fix inability to switch region via Switch Region dropdown

Prevent logged-in users redirect in case 'login'
view is used for switching the regions via modal form.

Change-Id: I47f26eea19e577998c7e3906a51900b51024eb43
Related-Bug: #1381413
Merge "Set default auth_url from django settings when auth_url is None"
Merge "Fix inability to switch region via Switch Region dropdown"