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/inc/python b/inc/python
index d9451b4..dfc4d63 100644
--- a/inc/python
+++ b/inc/python
@@ -15,6 +15,13 @@
 set +o xtrace
 
 
+# Global Config Variables
+
+# PROJECT_VENV contains the name of the virtual enviromnet for each
+# project.  A null value installs to the system Python directories.
+declare -A PROJECT_VENV
+
+
 # Python Functions
 # ================
 
@@ -105,7 +112,6 @@
                 -r $test_req
         fi
     fi
-    $xtrace
 }
 
 # get version of a package from global requirements file