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.
YAML is a superset of JSON, so maybe you could marshal to json and then run it through a yaml pretty-printer? Or just json.Marshal, yaml.Unmarshal into a `map[interface{}]interface{}`, then yaml.Marshal again. It's kind of ridiculous, but so is grepping for xxx_unrecognized.