Configurable Dynamic Themes

Follow on functionality for Dynamic Themes, v1:
https://blueprints.launchpad.net/horizon/+spec/horizon-dynamic-theme

Abstract:
Currently, Horizon only supports themes as configured by settings.py and local_settings.py which configures which theme to use by the setting of a COOKIE value. There has been desire to make this functionality more configurable from a deployment stand point, i.e. setting a theme based on Project/Domain/Cloud/Region or even policy.

Plan of Attack:
If the AVAILABLE_THEMES is configured to take a list/tuple of functions (much like other Django settings: TEMPLATE_CONTEXT_PROCESSORS, etc), then the theme that is returned can use more than simple COOOKIE logic. The current functionality (the use of COOKIES) will be encapsulated into a function that people can use or replace as they see fit.