Document a way to see masked fields in output

Right now ironic masks some fields in driver_info (ipmi_password) and in driver_internal_info (config_drive, image_url), etc.

This is very clever and right, but occasionally operators need those fields to debug issues. For example, if server stopped to respond to IPMI commands it's obvious for operators to try those command manually, as well, as attempt to download image if failed to download, etc.

To get this masked information operators may use database queries (select driver_info from nodes where uuid='37522db5-298c-41c4-9e6b-88382698a843';) but this is rather tedious and dangerous.

I propose to add some knob to specifically ask server not to mask those fields. Obviously it should be available only to administrators.

For CLI it may be something like 'ironic node-show node-name --unmask'.