Haproxy service check

Add service checks to Haproxy.
These service checks will be tagged by the service name (eg 'service:dogweb_query') and the backend (eg 'backend:i-abcdef')
They will report OK, CRITICAL and UNKNOWN values, depending on the status parsed, so they can be used to replace the events that are already existing.

The events are not removed to prevent removing them from people not having service_checks in place,but they can be removed by setting the `status_check` configuration flag to false.
line 18 it's no_check instead of no check
could you check in the doc what's the real value and update either one or the other?