add new command line handler

add new command line handler

New command line handler with pluggable classes.

Task subcommand added.

To run a scenario: yardstick -d task start samples/ping.yaml

$ yardstick -h
usage: yardstick [-h] [-V] [-d] [-v] {task} ...

Command-line interface to yardstick

optional arguments:
  -h, --help     show this help message and exit
  -V, --version  display version
  -d, --debug    increase output verbosity to debug
  -v, --verbose  increase output verbosity to info

subcommands:
  {task}

$ yardstick task -h
usage: yardstick task [-h] {start} ...

Task commands.

       Set of commands to manage benchmark tasks.

optional arguments:
  -h, --help  show this help message and exit

subcommands:
  {start}

$ yardstick task start -h
usage: yardstick task start [-h] [--keep-deploy] [--parse-only]
                            [--output-file OUTPUT_FILE]
                            taskfile

Start a benchmark scenario.

positional arguments:
  taskfile              path to taskfile

optional arguments:
  -h, --help            show this help message and exit
  --keep-deploy         keep context deployed in cloud
  --parse-only          parse the benchmark config file and exit
  --output-file OUTPUT_FILE
                        file where output is stored, default
                        /tmp/yardstick.out

JIRA :-
Signed-off-by: Jo¶rgen Karlsson <jorgen.w.karlsson@ericsson.com>
Change-Id: If0672594efa4c94c94ebb73f0bc97ecfe3e00c62