Virtual environment groundwork

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
diff --git a/clean.sh b/clean.sh
index fea1230..4f8c051 100755
--- a/clean.sh
+++ b/clean.sh
@@ -120,7 +120,7 @@
 fi
 
 # Clean up venvs
-DIRS_TO_CLEAN="$WHEELHOUSE"
+DIRS_TO_CLEAN="$WHEELHOUSE ${PROJECT_VENV[@]}"
 rm -rf $DIRS_TO_CLEAN
 
 # Clean up files