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/stack.sh b/stack.sh
index d83952a..9d4a206 100755
--- a/stack.sh
+++ b/stack.sh
@@ -702,6 +702,7 @@
 
 if is_service_enabled $DATABASE_BACKENDS; then
     install_database
+    install_database_python
 fi
 
 if is_service_enabled neutron; then