Should be able to add simple HORIZON_CONFIG entries

[Mathieu Gagné]

We should be able to add simple HORIZON_CONFIG entries without having to override the whole template.

Example of use case:
http://docs.openstack.org/developer/horizon/topics/customizing.html

HORIZON_CONFIG = {
    "customization_module": "my_project.overrides"
}

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/129724

[OpenStack Infra (hudson-openstack)]

Change abandoned by Mathieu Gagné (<email address hidden>) on branch: master
Review: https://review.openstack.org/129724
Reason: I think this change can be abandoned since we are now using concat to build local_settings.py, allowing anyone to add arbitrary fragments to the file.