Add install_venv library.

This adds a install_venv.py library that can be used by each
project's tools/install_venv.py script.

Currently all the projects have very similar install_venv
scripts in their respective tools directories that all
accomplish the same end result. This just breaks out the
common code and puts it in a class that can be imported
and used in each project's script.

Change-Id: I8346fc4201f35e5d0c475caef893ca71e7db8236
Merge "Add install_venv library."