pkt_gen: Adding Support of testcenter traffic generation using STC REST API.

pkt_gen: Adding Support of testcenter traffic generation using STC REST API.

Stcrestclient package based script is added. Support for RFC22544
backtoback, latency, frameloss are also added, along with the throughput.

Comments Addressed:
1.  Parameter name consistency. Ex: use duration (refer dummy.py).
2.  Moved 'only the necessary contents of README to
    configguide/trafficgen.rst.
3.  Removed README file
4.  Commit message format (pkt_gen: and linewrap)
5.  Merged 4 scripts (throughput/backtoback/latency/frameloss)
    that were rest-based into one single script.
    Name: testcenter-rfc2544-test.py
6.  Changed year to 2016
7.  Moved imports to the top (testcenter-rfc2544-rest.py)
8.  Changed prints to Log (testcenter-rfc2544-rest.py)
9.  Reporting the frame loss percentage.
10. Calls to frameloss and latency are not added as the 'caller'
    class TrafficControllerRFC2544 is only calling backtoback and
    throughput.
11. Changed Exception, e to Exception as e for python 3
    compatibility
12. Remaining prints to debug and logger.error for errors.
13. Moved Session creation under try block.
14. Removed deadcode
15. Fixed missing docstring
16. Generic to specific Exception.
17. if-if to if-elif
18. Debug level logging only under verbose flag.
19. Fixed Arguments mismatch.
20. Ordering of the imports
Note: The testcenter-rfc2544-rest.py is spawned as a separate process,
and need not be wrapped in a class.

JIRA: VSPERF-237

Change-Id: I05d2b24fca1da4ba67bc4d79206c5d755ebf8f20
Signed-off-by: sridharkn <sridhar.rao@spirent.com>