allow HAProxy to bind to a non-local IP address

This is needed since HAProxy will be running on each of our
load-balancer nodes, yet keepalived will ensure that our virtual IP
address exists on only one of our load-balancer nodes at any given time.

Signed-off-by: Emilien Macchi <emilien.macchi@enovance.com>
Merge pull request #509 from enovance/haproxy-binding

allow HAProxy to bind to a non-local IP address