Switch admin REST API to always return configs as JSON.

Convert to and from YAML for command-line usage only when setting
and getting ZoneConfigs. JSON seems more appropriate for use through
HTTP REST API.

Elide "xxx_unrecognized: []" entries from YAML output via a regexp
search and replace.
This is cleaner that what I was trying, which was splitting the string by \n, then joining it omitting any lines that had xxx_unrecognized: [] in them.