trex_prom: Add promiscuous option for trex for SRIOV tests

trex_prom: Add promiscuous option for trex for SRIOV tests

Adds option to enable promiscuous mode for Trex traffic
generator to allow for SR-IOV tests to work correctly.
Trex by default only accepts packets based on destination
MAC address matching what is configured on server in
/etc/trex-cfg.yaml. This removes that limitation since
SR-IOV tests require mac addresses to be modified in the
generated packets to correctly route packets to the relevant
VFs.

JIRA: VSPERF-531

Change-Id: I19b64f364c8a1b656994c82c96f47a68b92486d8
Signed-off-by: Christian Trautman <ctrautma@redhat.com>