[WORKER] Return node status in STATS message

The message returned to the statsd daemon will now contain a
list of all nodes defined in the haproxy.cfg file and their status.
Status is currently as reported from HAProxy (UP/DOWN/no check/etc.)

Fields added to the returned JSON response:

 "nodes": [ { "id": "1234", "status": "UP" } ]

The 'id' part of the JSON response will contain either the unique
ID of the node (as defined during the UPDATE message), or "serverN"
for older configs that haven't been updated to use the node ID.

Change-Id: I794b4901f542d922ebccd9ece9d86236584d4b46