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.
I tried the json->interface{}->yaml and the output was horrific. The yaml protobuf opts I have seem to do a pretty good job with the yaml...all with the exception of the xxx_unrecognized. Anyway, this will do for now.