Setting Pip Version for Virtual Environments

[GusPriceAtlassian]

This is not so much a bug as a feature request. My goal is to set the pip version to a specific version in virtual environments, instead of always creating virtualenvs with "--upgrade pip". I created the following PR to address this:

#558

I'm not sure that this is necessarily the optimal way to get this kind of input, but by adding this variable, when I declare virtualenvs I can now pass a pip version in the format "pip==9.0.3".