Make haproxy package management optional

Puppetlabs-haproxy does not use ensure_resources
and can cause failures when combined with lbaas
agent class.

This patch makes package management of haproxy
optional so that agents::lbaas can play nicely
with that module without requiring a patch there
changing to ensure_resource.

Change-Id: I6e603cf328ade9460e98e391ef01337fcb6ea72b