Add missing vpnaas class in example manifest

examples/neutron was missing neutron::agents::vpnaas which is useful
for people wanting to write manifests based on puppet-neutron module.

Change-Id: I374c4315472d46397c1cc2152bd35e1cdfd7336a
Signed-off-by: Emilien Macchi <emilien.macchi@enovance.com>
(cherry picked from commit 37d1c5b81087d241a26a6ee84278fb0b9bf1eabe)