rebuild the tempest tox env during install

When redoing a stack.sh over and over again, tempest venv can get out
of sync and cause issues until deleted. We should rebuild that tempest
venv on every stack.

Change-Id: I2f66bb1a7ccf9f89e11db1326d8553589e52fbf2
diff --git a/lib/tempest b/lib/tempest
index cf7eb6f..1160ee7 100644
--- a/lib/tempest
+++ b/lib/tempest
@@ -618,7 +618,7 @@
     git_clone $TEMPEST_REPO $TEMPEST_DIR $TEMPEST_BRANCH
     pip_install tox
     pushd $TEMPEST_DIR
-    tox --notest -efull
+    tox -r --notest -efull
     # NOTE(mtreinish) Respect constraints in the tempest full venv, things that
     # are using a tox job other than full will not be respecting constraints but
     # running pip install -U on tempest requirements