Python module argparse has a feature to load arguments from a file.
This commit enables such feature on sanitycheck script so files
prefixed by + (plus sign) are loaded during argument parsing stage
and included as arguments to such script.
Jira: ZEP-122
Change-Id: Ibb0a6e8464218a4f58ce4044a5a0860dc5b32480
Signed-off-by: Genaro Saucedo Tejada <genaro.saucedo.tejada@intel.com>