How do I retrieve a specific photo from iCloud backup of my camera roll
I guess with videos and Photo Stream you're out of luck.  

From the iCloud: Photo Stream FAQ:  


Does Photo Stream support video?


No. Photo Stream only works with photos.



With a local backup you could use something like the iOS Backup Extractor to extract the iOS files, including your video.  

Don't know about iCloud backups and extraction of specific parts.

Good luck.