Lists devices which have been registered.
Lists devices registered with /devices/register. Devices will age out of list after some amount of time. Used by clients to find dynamic resouces.
Example #1 - Query list of devices: Url: /devices/list
Query: No Content
Response: { "weatherstation.jszrc": { "ip": "10.13.0.222" "lastupdatesec": 1515260305 }, ... }