Sync venv scripts from oslo-incubator

Currently install_venv.py and with_venv.sh create virtual
environment in .venv directory and there is no way to specify
the virtual environment path explicitly.

This has been fixed in oslo-incubator.
This patch sync those virtual environment scripts from oslo-incubator.
Also make run_test.sh and run_tempest.sh scripts to use 'VENV' env variable.

Change-Id: I55311cff4dea1f6c5adaa2a6d51eeee2f6cc71ea
Closes-Bug: 1370086
diff --git a/tools/with_venv.sh b/tools/with_venv.sh
index 550c477..165c883 100755
--- a/tools/with_venv.sh
+++ b/tools/with_venv.sh
@@ -1,4 +1,6 @@
 #!/bin/bash
-TOOLS=`dirname $0`
-VENV=$TOOLS/../.venv
-source $VENV/bin/activate && "$@"
+TOOLS_PATH=${TOOLS_PATH:-$(dirname $0)/../}
+VENV_PATH=${VENV_PATH:-${TOOLS_PATH}}
+VENV_DIR=${VENV_DIR:-/.venv}
+VENV=${VENV:-${VENV_PATH}/${VENV_DIR}}
+source ${VENV}/bin/activate && "$@"