Adds usage examples for CONFIG_NEUTRON_L2_PLUGIN

In a previous patch, I did document it in docs/packstack.rst. It should
be documented correctly in the answer file too !!!!

Change-Id: I1cc6e555deac4da371fda1de862f987b37c4cdfb
Fixes: rhbz#1066019
Signed-off-by: Gael Chamoulaud <gchamoul@redhat.com>