This change uses virtualenv to insulate us from platform differences
in our building of the Python SDK, and to create an isolated Python 2
environment. This includes meaning we don't need to worry about the
specific location and behavior of Pylint. I *think* this will work
no matter whether it's Mac, Ubuntu, ArchLinux, Windows, and so on.
We do install to the --user directory in the install target using
`pip install -e`, however, which enables the machine-wide symlinking
that we need to support various workflows.
This fixespulumi/pulumi#1007.
