/// <summary>
 /// Add roles to user's list of roles.
 /// </summary>
 /// <param name="policy">admin configuration parameters, pass in null for defaults</param>
 /// <param name="user">user name</param>
 /// <param name="roles">role names.  Predefined roles are listed in Role.cs</param>
 public void GrantRoles(AdminPolicy policy, string user, IList<string> roles)
 {
     AdminCommand command = new AdminCommand();
     command.GrantRoles(cluster, policy, user, roles);
 }