Add option to allow user to pass vpe_script

Add option to allow user to pass vpe_script

Add option to allow user to pass their own vpe_script

vpe_script will be auto-generated by default.

To use your own config file use the 'script' parameter in the config file

vnf_config: { file: './vpe_config/vpe_config_2_ports',
              action_bulk_file: './vpe_config/action_bulk_512.txt',
              full_tm_profile_file: './vpe_config/full_tm_profile_10G.cfg',
              script_file: './vpe_config/vpe_script_sample }

JIRA: YARDSTICK-1525

Change-Id: Ie23e5705f4c0475e858be73b8a504a7df8898828
Signed-off-by: John O Loughlin <john.oloughlin@intel.com>