engine: Run tox jobs in parallel

Change-Id: Ied53d2e4cfb71f1a863dff71a3fc075677f921d0
diff --git a/jjb/engine/scripts/engine-verify-tox.sh b/jjb/engine/scripts/engine-verify-tox.sh
index 0d0870f..be8b846 100755
--- a/jjb/engine/scripts/engine-verify-tox.sh
+++ b/jjb/engine/scripts/engine-verify-tox.sh
@@ -64,9 +64,10 @@
   redirect_cmd source .venv/bin/activate
   set -u
 
-  # install test-requirements
-  echo "Info  : Install python packages listed in test-requirements.txt using pip"
-  redirect_cmd pip install --force-reinstall -r test-requirements.txt
+  # install only tox since the rest of the requirements are installed by tox itself
+  TOX_PACKAGE=$(grep "^tox==" test-requirements.txt)
+  echo "Info  : Install $TOX_PACKAGE"
+  redirect_cmd pip install --force-reinstall "$TOX_PACKAGE"
 
   # run tox
   echo "Info  : Run $LINT_TYPE using tox"