Add global venv enable/disable knob

Adds USE_VENV to globally enable/disable use of virtual environments.

ADDITIONAL_VENV_PACKAGES is used to manually add packages that do not
appear in requirements.txt or test-requirements.txt to be installed
into each venv.  Database Python bindings are handled this way when
a dataabse service is enabled.

Change-Id: I9cf298b936fd10c95e2ce5f51aab0d49d4b7f37f
diff --git a/lib/stack b/lib/stack
index 9a509d8..11dd87c 100644
--- a/lib/stack
+++ b/lib/stack
@@ -16,13 +16,17 @@
 function stack_install_service {
     local service=$1
     if type install_${service} >/dev/null 2>&1; then
-        if [[ -n ${PROJECT_VENV[$service]:-} ]]; then
+        if [[ ${USE_VENV} = True && -n ${PROJECT_VENV[$service]:-} ]]; then
             rm -rf ${PROJECT_VENV[$service]}
-            source $TOP_DIR/tools/build_venv.sh ${PROJECT_VENV[$service]}
+            source $TOP_DIR/tools/build_venv.sh ${PROJECT_VENV[$service]} ${ADDITIONAL_VENV_PACKAGES//,/ }
             export PIP_VIRTUAL_ENV=${PROJECT_VENV[$service]:-}
+
+            # Install other OpenStack prereqs that might come from source repos
+            install_oslo
+            install_keystonemiddleware
         fi
         install_${service}
-        if [[ -n ${PROJECT_VENV[$service]:-} ]]; then
+        if [[ ${USE_VENV} = True && -n ${PROJECT_VENV[$service]:-} ]]; then
             unset PIP_VIRTUAL_ENV
         fi
     fi