Improved a virtualenv option

Replaced the "--system-site-packages" option of virualenv with "--no-site-packages" option, so that the packages required by tensorflow are installed separately with the system packages.
 Otherwise, there could be multiple versions of an installed package (e.g. numpy-1.8.2 installed by apt and numpy-1.12.0 installed by pip), and an error may occur when executing "import tensorflow".