selftests: openvswitch: add interface support

Includes an associated test to generate netns and connect
interfaces, with the option to include packet tracing.

This will be used in the future when flow support is added
for additional test cases.

Signed-off-by: Aaron Conole <aconole@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>