Cap tox != 2.8.0 temporarily to avoid ValueError

This is proposed temporary change to avoid
"ValueError: need more than 1 value to unpack" error in stack.

Change-Id: I743febbef3a1f201cea37471356518be31585277

Author: Anna Khmelnitsky

lib/tempest

@@ -608,7 +608,7 @@ function configure_tempest {
 # install_tempest() - Collect source and prepare
 function install_tempest {
     git_clone $TEMPEST_REPO $TEMPEST_DIR $TEMPEST_BRANCH
-    pip_install tox
+    pip_install 'tox!=2.8.0'
     pushd $TEMPEST_DIR
     tox -r --notest -efull
     # NOTE(mtreinish) Respect constraints in the tempest full venv, things that