examples: Add clean-traffic-gateway into nwfilters

examples: Add clean-traffic-gateway into nwfilters

The filter purpose is to simulate isolated private VLAN.

The behavior can be achieved by limiting network traffic
to traffic between VM and gateway. Because there is no
concept of the PVLAN in the linux bridge.

The filter also contains parts from clean-traffic
to prevent VM from spoofing its IP and MAC address.

To use this filter the user just needs to set
the GATEWAY_MAC variable to gateway MAC address.

Signed-off-by: Ales Musil <amusil@redhat.com>
Reviewed-by: Martin Kletzander <mkletzan@redhat.com>