Added CORS support to Mistral

[OpenStack Infra]

https://review.openstack.org/265370
Dear bug triager. This bug was created since a commit was marked with DOCIMPACT.
Your project "openstack/mistral" is set up so that we directly report the documentation bugs against it. If this needs changing, the docimpact-group option needs to be added for the project. You can ask the OpenStack infra team (#openstack-infra on freenode) for help if you need to.

commit b57e1b0286eea8797dfd92523672b716136e3163
Author: Michael Krotscheck <email address hidden>
Date: Fri Jan 8 09:33:27 2016 -0800

    Added CORS support to Mistral

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

    For Mistral, the CORS middleware was added directly to the wsgi factory.
    Config generation and tests have also been provided.

    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: Add link to CORS configuration in admin cloud guide.

    Change-Id: Ic8f2e257acb256d9ea99b66d2ed1dcfc8652ae89