Use glint to determine if os.Stdout is a terminal.

glint Terminal renderer expects os.Stdout [not only to be a terminal, but also to have non-zero size](https://github.com/mitchellh/go-glint/blob/b492b545f6208fc979e7ee498d55599b8107e770/renderer_term.go#L39-L46). It's unclear how this condition arises, but this additional check causes Nomad to render deployments progress through glint when glint cannot support it.

By using golint to perform the check, we eliminate the risk of mis-judgement.