Sets l2population to true on nodes that populate l2 agents

Change-Id: Idcd1bd8566e78bf16ee6299906dcc1d5415efc13
Fixes: rhbz#1118010
Signed-off-by: Gael Chamoulaud <gchamoul@redhat.com>
Depends-on: https://github.com/redhat-openstack/openstack-puppet-modules/pull/81