Change CORS headers to be assigned to containers

Copy content from
http://docs.openstack.org/developer/swift/cors.html
to replace misleading content in
http://docs.openstack.org/api/openstack-object-storage/1.0/
content/assigning-cors-headers-to-requests.html

Change "&gt;" to ">" in example

Use $publicURL to represent the endpoint which includes
the API version number and the account/tenantID.

Closes-Bug: #1260840

Change-Id: I9dfe46d610bfcf2f9646cb28a4246954c180252d