Use python3 venv instead of virtualenv

tempest-tox-plugin-sanity-check job is failing due to missing virtualenv.
As the job is running on master only and we don't support python2 anymore,
this patch edits the tempest-plugin-sanity.sh script so that it uses
python3 venv module to create a venv.

Change-Id: I3f711ec97ad9006c715bb45db9d8dd9d951bb6a1
diff --git a/tools/tempest-plugin-sanity.sh b/tools/tempest-plugin-sanity.sh
index 2ff4aea..c983da9 100644
--- a/tools/tempest-plugin-sanity.sh
+++ b/tools/tempest-plugin-sanity.sh
@@ -66,7 +66,7 @@
 # function to create virtualenv to perform sanity operation
 function prepare_workspace {
     SANITY_DIR=$(pwd)
-    virtualenv -p python3 --clear "$SANITY_DIR"/.venv
+    python3 -m venv "$SANITY_DIR"/.venv
     export TVENV="$SANITY_DIR/tools/with_venv.sh"
     cd "$SANITY_DIR"