code: func (u *UserCommand) Update(client auth.ClientI) error {
  	user, err := client.GetUser(u.login)
  	if err != nil {
  		return trace.Wrap(err)
  	}
  	roles := strings.Split(u.roles, ",")
  	for _, role := range roles {
  		if _, err := client.GetRole(role); err != nil {
  			return trace.Wrap(err)
  		}
  	}
  	user.SetRoles(roles)
  	if err := client.UpsertUser(user); err != nil {
  		return trace.Wrap(err)
  	}
  	fmt.Printf("%v has been updated with roles %v\n", user.GetName(), strings.Join(user.GetRoles(), ","))
  	return nil
  }
docs: // Update updates existing user