Display senstive config values not in clear text

[Emily-Jiang]

As an operations person, I want a way to define parts of the config whose values will not printed out in clear text but only as e.g. '**********' Sometimes config is also used for critical information like secret data which must not show up in logs etc.

[tomas-langer]

This will probably require some kind of per-property meta information (to mark it as a secret). I think we should decide how to handle this asap and factor it into the spec