Multi-region support in shared Keystone service deployment

Summary
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Multi-region support in shared Keystone service deployment.

Motivation
==========
As a public cloud provider(Catalyst Cloud) based on OpenStack, we deployed Keystone as a shared identity service across 3 regions, our customers have been asking for the feature that they could select their preferred region when they log in Horizon, rather than switching region each time after login.

Description
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A new setting named 'OPENSTACK_KEYSTONE_REGION_CHOICES' is introduced to Horizon config file, which is supposed to be used in shared-keystone, multi-region deployment without breaking the existing 'AVAILABLE_REGIONS' usage, which only works with multi- keystone, multi-region environment. It's going to be configed by cloud operators and provides end user the ability to specify region when logging in Horizon. For backward compatibility, the 'AVAILABLE_REGIONS' setting takes precedence over 'OPENSTACK_KEYSTONE_REGION_CHOICES'.

UX
===
N/A

Testing
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Give there are 3 regions (Region1, Region2, Region3) in a shared-keystone, multi-region OpenStack deployment. with all the changes applied, operator could config:

OPENSTACK_KEYSTONE_REGION_CHOICES = ['Region1', 'Region2', 'Region3']

End user should see all regions in a dropdown box when accessing Horizon and select her preferred one to log in.

Outside Dependencies
====================
N/A

Requirements Update Required
============================
The main code change is in django-openstack-auth repo, once it's merged we need to bump django-openstack-auth version in global-requirements and update Horizon requirments.txt accordingly.

Doc Impact
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The usage of 'OPENSTACK_KEYSTONE_REGION_CHOICES' should be described.