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