yardstick: CLI params support for yardstick TCs

yardstick: CLI params support for yardstick TCs

Vswitchperf specific class in yardstick was updated to reflect
new functionality of --test-params. These changes have impact
on vsperf documentation and yardstick samples.
As part of this fix following changes were made:
* trafficgen mode can be executed with default configuration
* yardstick specific TC samples were updated
* dummy traffic generator was enhanced to support back2back
* dummy traffic generator was enhanced to accept traffic
results from configuration

JIRA: VSPERF-422
JIRA: VSPERF-205

Change-Id: Ibeaa6ef46bde453e3ca5dee6e4568fc8c3853521
Signed-off-by: Martin Klozik <martinx.klozik@intel.com>
Reviewed-by: Al Morton <acmorton@att.com>
Reviewed-by: Christian Trautman <ctrautma@redhat.com>
Reviewed-by: Bill Michalowski <bmichalo@redhat.com>
Reviewed-by: Antonio Fischetti <antonio.fischetti@intel.com>
Reviewed-by: <sridhar.rao@spirent.com>