[Fullstack] Additional log of tcpdump stderr output
[Fullstack] Additional log of tcpdump stderr output

This patch adds additional logging of tcpdump stderr output in
DSCP marking packets tests.
It is required to figure out why those tests are failing sometimes.

Change-Id: I02fe64bf817b47970e4e073f48ea8dea53bdd4f5
Related-Bug: #1733649

mahak/neutron,mahak/neutron,huntxu/neutron,mahak/neutron,huntxu/neutron,openstack/neutron,noironetworks/neutron,noironetworks/neutron,openstack/neutron,openstack/neutron