Implement rate limiting (#43)

* Implement ratelimiting
- by sending back '429 Too Many Requests' error message
- Error message is a HTTP/1.1 response, so contrail link local proxy works with it too.
- HAProxy sends back HTTP/1.0 responses, so using a template is required.

Only possible when using front-/backend configuration layout.

* Add options in frontend and backend.

Options are filtered by a blacklist of invalid options, as per haproxy manual.

* Add sls test for rate limiting configuration

* Update readme with rate limiting example
6 files changed