Fix use of venv in Tempest.

Currently when running Tempest tests with run_tests.sh a venv
isn't being used. This leaves tempest at the mercy of whatever
required package versions are installed on the system.

This patch takes install_venv.py and with_venv.sh from nova and
integrates them into run_tests to ensure that the correct
dependencies versions from pip-requires and test-requires are
being used.

Change-Id: I4bf4a02890a33c4034e4493d1763ed4019fdf46e
diff --git a/tools/with_venv.sh b/tools/with_venv.sh
index 2e2b855..550c477 100755
--- a/tools/with_venv.sh
+++ b/tools/with_venv.sh
@@ -1,4 +1,4 @@
 #!/bin/bash
 TOOLS=`dirname $0`
-VENV=$TOOLS/../.kong-venv
-source $VENV/bin/activate && $@
+VENV=$TOOLS/../.venv
+source $VENV/bin/activate && "$@"