ADMIN_API: Fixes to make admin_api work with current tools

* Hack to get 'usage' routing working
* Devices list output needed to be in 'devices' dict
* PUT/POST don't wait on gearman so should return 200 instead of 202
* Pool MGM and statsd need to specify json content (WSME is stricter
  than the Java thing we used)

Change-Id: I2fef129ef81045d7b7929e52f09670608c47d801