Support for Multidomain mapping

Summary
=======
With this feature will going to allow to the administrators set a domain mapping for each hostname.

Motivation:
=========
This change will going to allow the openstack administrators allow to manage multiple domains based on the "HOST HEADER" value.

Description
==========

When this patch is applied the end user doesn't need to know what domain he/she need to login.
For example we can map if a user access to horizon from:

https://www.myopenstack.com the field DOMAIN in horizon will be prefilled with the Domain Information. in this example the domain is MYOPENSTACK.

The openstack administrator also can configure multiple hostnames to diferents domains
Example 2
Hostname Prefilled Domain Field
www.hello.com ===> HI
infra.mysite.com ===> MYDOMAIN2

UX
==
No visual changes.

Testing
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After this patch is applied users can modify in local_settings.py the domain mapping with the variable OPENSTACK_KEYSTONE_MULTIDOMAIN_MAPPING = { 'www.mysite.com' : 'MYDOMAIN',}

Outside Dependencies
===================
Required modify DOA to allow this functionality.