Make venv dir name OS specific

This is to allow the Vagrantbox to mount your local dev on a Mac, yet
have venv working even when it includes OS-specific binary files.
diff --git a/env.sh b/env.sh
index e88f121..0e29340 100644
--- a/env.sh
+++ b/env.sh
@@ -1,8 +1,16 @@
 # sourcing this file is needed to make local development and integration testing work
 export VOLTHA_BASE=$PWD
 
-# load local python virtualenv 
-. venv/bin/activate
+# load local python virtualenv if exists, otherwise create it
+VENVDIR="venv-$(uname -s | tr '[:upper:]' '[:lower:]')"
+if [ ! -e "$VENVDIR/.built" ]; then
+    echo "~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~"
+    echo "Initializing OS-appropriate virtual env."
+    echo "This will take a few minutes."
+    echo "~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~"
+    make venv
+fi
+. $VENVDIR/bin/activate
 
 # assign DOCKER_HOST_IP to be the main ip address of this host
 export DOCKER_HOST_IP=$(python voltha/nethelpers.py)