pip module: fix unexpected behavior when creating virtualenvs

This module supports `virtualenv_site_packages` as an option to control whether or not to include system packages when installing pip packages into a virtual environment. 

The default is no, and when you say yes it explicitly it includes a flag to include site packages, otherwise it doesn't provide a flag. 

Some versions of virtualenv by default include site packages and other versions by default do not. 

This patch just makes both branches explicit, so `virtualenv_site_packages=no` really means no.
Merge pull request #4735 from chrisalbright/patch-1

pip module: fix unexpected behavior when creating virtualenvs