This patch adds haproxy package to devstack as Neutron will rely on it
for serving metadata instead of the current Python implementation.
haproxy will reduce the memory footprint from ~50MB to ~1.5MB for
serving metadata. It will be spawned for every Neutron router so,
for large deployments, it will be a significant memory reduction.
Change-Id: I36a5531cacc21c0d4bb7f20d4bec6da65d04c262