xena: Throughput method implementation for Xena Networks

xena: Throughput method implementation for Xena Networks

Add files for Xena Networks 2544 throughput
implementation. This allows for Xena to execute
tput tests from VSPerf. For information on how
this works please see the JIRA ticket.

Modify traffic gen guide on install instructions
for Xena. Add in scapy to requirements file. Modify
conf file for Xena chassis connection information.

* Added license info to __init__.py / baseconfig.x2544
* Set initial rate to 10% line rate for tput testing
* Modified default mac address xena_json->set_header_layer2
  from aa:aa:aa:aa:aa:aa to cc:cc:cc:cc:cc:cc

* Removed obsolete multistream params
* Added traffic params to 03_traffic.conf
* Comments to explain try/catch for latency values
* Removed unused import

* Rebased to fix merge conflict
* Added Xena to available traffic gen list in conf files

JIRA: VSPERF-261

Change-Id: I3c4b4a332fd532e10b6096e9ed97a4a39d410839
Signed-off-by: Christian Trautman <ctrautma@redhat.com>