Adds usage examples for CONFIG_NEUTRON_L2_PLUGIN

Change-Id: I5371c0510ddb7b16e1cf1ce07998a02f531a5bef
Fixes: rhbz#1066019
Signed-off-by: Gael Chamoulaud <gchamoul@redhat.com>
Merge "Adds usage examples for CONFIG_NEUTRON_L2_PLUGIN"