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

lib/database

@@ -109,6 +109,11 @@ function install_database {
     install_database_$DATABASE_TYPE
 }
 
+# Install the database Python packages
+function install_database_python {
+    install_database_python_$DATABASE_TYPE
+}
+
 # Configure and start the database
 function configure_database {
     configure_database_$DATABASE_TYPE