[rackspace|cdn] updated to re-authenticate and retry request when auth token expires

Developers are reporting receiving a HTTP 401 - Unauthorized using the Rackspace Cloud with carrierwave. This issue appears to be occurring because the CDN service is not re-authenticating after its authentication token expires.

For more information please refer to http://stackoverflow.com/questions/17139148/http-401-fogstoragerackspaceserviceerror/17149654#17149654

Note, this pull request only addresses the CDN service. I am going to determine if this issue exists for other Rackspace services and if necessary open up a separate PR to address those.