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>
(cherry picked from commit 0d7db88e83b29e45d797307a11ef18c1438ab02b)
Merge "Adds usage examples for CONFIG_NEUTRON_L2_PLUGIN" into icehouse