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/stackrc b/stackrc
index 02b12a3..3c4593a 100644
--- a/stackrc
+++ b/stackrc
@@ -104,6 +104,16 @@
     source $RC_DIR/.localrc.auto
 fi
 
+# Enable use of Python virtual environments.  Individual project use of
+# venvs are controlled by the PROJECT_VENV array; every project with
+# an entry in the array will be installed into the named venv.
+# By default this will put each project into its own venv.
+USE_VENV=$(trueorfalse False USE_VENV)
+
+# Add packages that need to be installed into a venv but are not in any
+# requirmenets files here, in a comma-separated list
+ADDITIONAL_VENV_PACKAGES=${ADITIONAL_VENV_PACKAGES:-""}
+
 # Configure wheel cache location
 export WHEELHOUSE=${WHEELHOUSE:-$DEST/.wheelhouse}
 export PIP_WHEEL_DIR=${PIP_WHEEL_DIR:-$WHEELHOUSE}