lbaas: use stdlib to manage haproxy package

Instead of using Package resource to ensure HAproxy is installed, using
stdlib with ensure_packages would allow to declare haproxy class (from
another puppet module) without dupplication.

Closes-bug #1273285

Change-Id: I57c5dc8096c846b102dc8ef6451400bc025af056
Signed-off-by: Emilien Macchi <emilien.macchi@enovance.com>