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
diff --git a/README.rst b/README.rst
index 06fd886..09bcdd7 100644
--- a/README.rst
+++ b/README.rst
@@ -427,6 +427,30 @@
             port: 8443
             params: 'maxconn 256'
 
+Implement rate limiting, to prevent excessive requests
+This feature only works if using 'format: end'
+
+.. code-block:: yaml
+  haproxy:
+    proxy:
+      ...
+      listen:
+        nova_metadata_api:
+          ...
+          format: end
+          options:
+          - httpchk
+          - httpclose
+          - httplog
+          rate_limit:
+            duration: 900s
+            enabled: true
+            requests: 125
+            track: content
+          servers:
+            ...
+          type: http
+
 Read more
 =========