Add support for generating graphs from output

Add support for generating graphs from output

Command line tool yardstick-plot is to be used to visualize results
gathered from yardstick framework's output file. Currently supports
plotting graphs from ping, pktgen, iperf3 and fio tests.

Yardstick-plot takes two arguments - input file and output folder
and both of them have defaults to fall to if left unspecified.

Supports having multiple different scenario types in an input file,
while assuming that all results from the same scenario type belong
to one graph. Thus, results plotted from a single scenario type
with different parameters are currently non-informative.

yardstick-plot is declared as an extra for yardstick in setup.py as
it is not required for all use cases of the yardstick framework.
It can be installed for example using command:
    $ pip install -e .[plot]
from the folder where setup.py is located.

Example invocation: yardstick-plot -i /tmp/yardstick.out -o /tmp/plots/

JIRA: YARDSTICK-65

Change-Id: Ic436ca360ba2496aa829ca817b1d9d5f3c944c6c
Signed-off-by: Kristian Hunt <kristian.hunt@gmail.com>