openstack-full.install: Get haproxy from backports

Haproxy 1.5 is now present in wheezy-backports and not with a dev version.

https://packages.debian.org/wheezy-backports/haproxy

Signed-off-by: Dimitri Savineau <dimitri.savineau@enovance.com>
Merge pull request #111 from enovance/haproxy-from-backports

openstack-full.install: Get haproxy from backports