Provide CORS support for Magnum

[Michael]

This bug aims to capture the work necessary to provide CORS support in accordance to the OpenStack Cross-Project Spec outlined here: http://specs.openstack.org/openstack/openstack-specs/specs/cors-support.html

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/237199
Committed: https://git.openstack.org/cgit/openstack/magnum/commit/?id=cc713cd2d324ac290404b204bffb0bddb2ffc0ab
Submitter: Jenkins
Branch: master

commit cc713cd2d324ac290404b204bffb0bddb2ffc0ab
Author: Michael Krotscheck <email address hidden>
Date: Mon Oct 19 14:00:57 2015 -0700

    Added CORS support to Magnum

    This adds the CORS support middleware to Magnum, allowing a deployer
    to optionally configure rules under which a javascript client may
    break the single-origin policy and access the API directly.

    For magnum, the CORS middleware was directly added to the setup_app
    method, as the last middleware to be wrapped around the application
    instance. Common headers were added, in order to avoid magic
    configuration moments where a feature does not function without
    an operator having to read code.

    OpenStack CrossProject Spec:
       http://specs.openstack.org/openstack/openstack-specs/specs/cors-support.html
    Oslo_Middleware Docs:
       http://docs.openstack.org/developer/oslo.middleware/cors.html
    OpenStack Cloud Admin Guide:
       http://docs.openstack.org/admin-guide-cloud/cross_project_cors.html
    DocImpact

    Change-Id: I7e6d7e2145e3bf883b8a28071c949e10e5c207e0
    Closes-bug: #1508209