Rethink the backup option for Galera

Rethink the backup option for Galera

Initial logic for the backup option in the HAProxy was wrong and
wouldn't do what expected.

Current logic is implemented as follow

1. User passes an array of Galera IP addresses
2. User passes an array of Galera hostnames
3. User passes a Galera master IP and Galera master hostname

Result :

* Set a backend line with Galera master IP and Galera master hostname
* Remove those data from the two arrays of IP and Hostnames
* Set backend lines for whatever is left on those array with the backup
  option on

Change-Id: Idfd72de4fafdce2a9c16945961fee996a98049b7