added /controls endpoint for control_group (audit) messages

I don't see any common_tests in this repo, so testing will be via pedant in a separate PR.

See also:
- https://github.com/opscode/analytics_docs/pull/44
- https://github.com/opscode/opscode-omnibus/pull/594
- https://github.com/opscode/client-docs/blob/master/specifications/auditmode.md

cc @jamesc @sdelano @PrajaktaPurohit 
We could simplify this:

```
case ej:get({"node_name"}, ControlData) of
  ClientName -> 
    {authorized, Req, State};
  _Else ->
    {{halt, 403}, Req, State}
end;