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.
I did not modify this to mimic the comportment from the existing events and was not sure about the meaning of 'maint', which I guess is for maintenance but we may add it.

Regarding "no_check" vs. "no check", the relevant documentation is [here](http://cbonte.github.io/haproxy-dconv/configuration-1.5.html#9.1) and is quite unhelpful in that matter. From haproxy that I ran on my vm, I got "no check" so I will update the other value