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
2 files changed