Support direct from browser to searchlight results

CORS support allows browsers to directly contact a service. For example, with Horizon, we would have to go through the Horizon layers to get to Searchlight. With CORS support, we could have the javascript directly contact searchlight (assuming we figure out client side token handling with horizon).

References:
 http://docs.openstack.org/developer/oslo.middleware/cors.html

 http://specs.openstack.org/openstack/openstack-specs/specs/cors-support.html

 http://docs.openstack.org/developer/oslo.middleware/cors.html -> use version https://review.openstack.org/#/c/199761/ (actually, 2.5.0 is easier to debug)

Example for ironic https://review.openstack.org/#/c/199769/