Added CORS support to Neutron

Added CORS support to Neutron

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

For neutron, both the paste.ini and the direct-wrap method of
deploying the middleware were chosen, since neutron is in a state of
migration to pecan. Note that if someone deploys pecan-neutron behind
other middleware, any responses provided by that middleware will not
be CORS-enabled, as these responses would be returned before the
headers could be annotated. This results in a response not readable
by the user agent.

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: I02acea6124d28370d92e7c94ca2d1d6c5c8937ac