bugfix: Cleanup of traffic gen api and default traffic values

bugfix: Cleanup of traffic gen api and default traffic values

Both documentation and traffic generator API are updated
to match current code logic. Additional default values
are added into TRAFFIC_DEFAULTS dictionary. Constructor
of TestCase class uses default values from TRAFFIC_DEFAULTS
instead of magic numbers.

Change-Id: Id2f4983945e4b123adddbc9a2a832f4a735b27f6
JIRA: VSPERF-253
Signed-off-by: Martin Klozik <martinx.klozik@intel.com>
Reviewed-by: Maryam Tahhan <maryam.tahhan@intel.com>
Reviewed-by: Al Morton <acmorton@att.com>
Reviewed-by: Christian Trautman <ctrautma@redhat.com>