With the handler in the haproxy-config role, it gets triggered once for
every service that changes the firewall config. This happens because the
role is included dynamically. If we move the handler to the haproxy
role, which is only included once, the handler will trigger at most
once.

This is a follow up for Iea3680142711873984efff2b701347b6a56dd355.

Change-Id: Iad9ed241026435085bc9a0f5802818010b47830f