Allow the "hidden" attribute on outputs, like it is on parameters

In order to manage Heat's output of sensitive information (e.g. passwords), it would be useful to allow output items to be flagged as hidden:

outputs:
  compute_instance_password:
    description: password for the compute instance
    value: { get_attr: [compute_instance, password]}
    hidden: true

This would allow for controlling the display/providing of sensitive information in the outputs section.