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.
oopsie on a test
Added option to specify YAML or JSON format for zone configs.
Merge pull request #117 from cockroachdb/spencerkimball/yaml-json-configs

Switch admin REST API to always return configs as JSON.