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/databases/mysql

@@ -151,9 +151,12 @@ EOF
     else
         exit_distro_not_supported "mysql installation"
     fi
+}
 
+function install_database_python_mysql {
     # Install Python client module
     pip_install MySQL-python
+    ADDITIONAL_VENV_PACKAGES+=",MySQL-python"
 }
 
 function database_connection_url_mysql {

lib/databases/postgresql

@@ -100,9 +100,12 @@ EOF
     else
         exit_distro_not_supported "postgresql installation"
     fi
+}
 
+function install_database_python_postgresql {
     # Install Python client module
     pip_install psycopg2
+    ADDITIONAL_VENV_PACKAGES+=",psycopg2"
 }
 
 function database_connection_url_postgresql {