Wargs are people with the ability to enter the minds of animals and perceive the world through their senses and even control their actions. The act of doing this is called "warging".
Will Bran Stark enter the mind of Drogon or Rhaegal (the 2 living dragons). On the other hand, a dragon is not a slave! I know the word slave is a bit much.
Better yet, will Bran Stark enter the mind of Viserion?
This is a good video on Viserion.