doc: Added CORS support to Aodh

https://review.openstack.org/265342
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commit ab70d21919464db0522b6ee82b9e18ad7aa72646
Author: Michael Krotscheck <email address hidden>
Date: Fri Jan 8 08:03:11 2016 -0800

    Added CORS support to Aodh

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

    In previous patches, proper paste support for gabbi was added. This patch
    modifies the paste.ini file for both aodh and gabbi, adds the CORS
    configuration fixture to support unit tests, and adds CORS into configgen.

    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: I27673b8581069c9ef02525756d1079b3904d8349