Return 200 in api/devices#register if ID already registered

API should return 200 if the device was already registered. This allows
us to skip checking of return values in client API bindings.
Accept more options in json_st...sponse in application

Allow the json_status_response to take an additional Hash to merge with
the returned default JSON. This allows us to extend this helper method
easily.
Load redis.yml through ERB, and then YAML

Allows us to use ERB-style substitutions in the redis YAML file, and
thus add environmental variables.