Enable traffic generator PID in "NSPerf" scenario setup

Enable traffic generator PID in "NSPerf" scenario setup

Now "NSPerf" scenario will inform about the traffic generator PIDs after
setup process. With this information, IterationIPC runner will be able
to receive the messages sent by those traffic generators and control
the main iteration loop.

The following example, using vFW as VNF and OpenStack as context, makes
use of this new runner implementation:
  /samples/vnf_samples/nsut/vfw/
    tc_heat_rfc2544_ipv4_1rule_1flow_64B_trex_iterationipc.yaml

JIRA: YARDSTICK-1127

Change-Id: I46b1368bc209680b88ff9fb5c3b9beadf6271ac9
Signed-off-by: Rodolfo Alonso Hernandez <rodolfo.alonso.hernandez@intel.com>