ixia: Make L3 and L4 headers optional

ixia: Make L3 and L4 headers optional

During testing it is sometimes useful to generate pure L2 traffic
without L3 or L4 related headers. IxNetwork TCL script was updated
to support this option.
Generic TRAFFIC array was enhanced by on/off switch for L3 and L4
headers. Thus it can be implemented by other traffic generators
in the future, if this feature will prove itself useful.

JIRA: VSPERF-500

Change-Id: I723c703e5d6ef609fd5b7db366871278a4730203
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: Sridhar Rao <sridhar.rao@spirent.com>
Reviewed-by: Trevor Cooper <trevor.cooper@intel.com>