add key access to private content to render thumbnails in cantaloupe

[kimpham54]

cantaloupe renders published content, needs to be able to render private content available through backend admin. add key access to generate thumbnails for backend

[jrynhart]

Check for api key param on /datastream route. Added a private index setting to configuration. Use the private index if a valid api key is present in a request to /datastream

[kimpham54]

done!