High Availability refactorization

* Implement sticky/persistent session with HAProxy. It implements sticky/persistent session betwee HAProxy and Horizon based on the leastconn fashion.
* Change httpchk options for Horizon. In order to have clean log in Apache2, we need to do a GET call instead of the default OPTIONS call. Also we specify a User Agent, for better filtering.
* Don't manage HAproxy service on BACKUP nodes.
* Fix Heat HAproxy pools
* remove the ```$*_internal``` parameters
* be able to specify multiple public VIPs in both ```$vip_public_ip``` and ```$vip_internal_ip```
* allow the ```$*_api``` parameters to be either:
*  ```true```: listen on both public & internal VIPs
*  ```false```: don't listen at all
* one or more IP addresses: this service is bound specifically to these VIPs (check later that these VIPs are declared either in ```$vip_public_ip``` or ```$vip_internal_ip```.

Credits to @Spredzy who initiated the work a few months ago.