loadbalancer: do not manage HAproxy service on BACKUP nodes

When a loadbalancer is in BACKUP mode, HAproxy can't bind the pools to
the VIP, since the VIP is not hosted. Puppet warns it when compiling in
the catalogue.
This patch ensure HAproxy is run where we Keepalived is in MASTER mode.

If a failover occurs and MASTER has changed, Keepalived & HAproxy are auto-migrated thanks to VRRP to another node. The worth case will only show warnings in Puppet catalog but HAproxy won't start anyway on the node.

Bug #237