Should stop using balena-sync

[thgreasi]

Since we are only using a small part of it and relying on a module with such name just for the discover functionality, we should move the implementation inside the CLI and drop the dependency to balena-sync (maybe later we could make a discover standalone module, but it doesn't have to be now). See: https://www.flowdock.com/app/rulemotion/i-cli/threads/UaLodwWrtDZI7fEl8mTumBks9xS