HAProxy: gauge missing statuses with zero

About count_per_status:
For each service, we currently only gauge the current status of the service.
This means that if the service was previously down and is up again, we are going to gauge(count, [up]) but not gauge(0, [down]).
Since on the UI the missing data is regressed, we can see this kind of bug:
![screen shot 2014-05-13 at 11 48 55 am](https://cloud.githubusercontent.com/assets/1532259/2959907/436a3d48-dab7-11e3-89c9-5b9a6f5b3159.png)
The second chart keeps displaying 2, instead of displaying 0 where the value is missing.

This PR is about solving that, by always gauging each status (either count or 0, depending on the current status).
Pros: avoid these bugs
Cons: more data pushed, charts with only zeros appear

New charts:
![screen shot 2014-05-13 at 11 47 18 am](https://cloud.githubusercontent.com/assets/1532259/2959945/9def64f0-dab7-11e3-98d5-25077d07d1be.png)

![screen shot 2014-05-13 at 11 47 27 am](https://cloud.githubusercontent.com/assets/1532259/2959942/97479fd2-dab7-11e3-8ebf-59b87f633096.png)