Introduce the tooling to build virtual environments.
* tools/build_venv.sh: build a venv
* introduce lib/stack to house functionality extracted from stack.sh that
is needed in other places, such as Grenade; start with stack_install_service
to wrap the venv install mechanics
* declare PROJECT_VENV array to track where project venvs should be installed
* create a venv for each project defined in PROJECT_VENV in stack_install_service()
Change-Id: I508588c0e2541b976dd94569d44b61dd2c35c01c
8c2ce6ea72