Added CORS support to Keystone
Added CORS support to Keystone

This adds the CORS support middleware to Keystone, allowing a deployer
to optionally configure rules under which a javascript client may
break the single-origin policy and access the API directly. Included
are Keystone's auth headers, so that anyone activating this
middleware does not have to explicitly enable them.

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

Change-Id: If58ad7d952ace9211f0ed8beff8552c216262382

maestro-hybrid-cloud/keystone,ajayaa/keystone,klmitch/keystone,mahak/keystone,jonnary/keystone,jonnary/keystone,ilay09/keystone,dims/keystone,himanshu-setia/keystone,openstack/keystone,openstack/keystone,ilay09/keystone,takeshineshiro/keystone,idjaw/keystone,cernops/keystone,klmitch/keystone,ajayaa/keystone,mahak/keystone,ilay09/keystone,idjaw/keystone,cernops/keystone,rajalokan/keystone,himanshu-setia/keystone,takeshineshiro/keystone,mahak/keystone,rajalokan/keystone,openstack/keystone,roopali8/keystone,maestro-hybrid-cloud/keystone,rajalokan/keystone,roopali8/keystone,dims/keystone