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/run_tempest.sh b/run_tempest.sh
index 5a9b742..0f32045 100755
--- a/run_tempest.sh
+++ b/run_tempest.sh
@@ -20,7 +20,7 @@
 }
 
 testrargs=""
-venv=.venv
+venv=${VENV:-.venv}
 with_venv=tools/with_venv.sh
 serial=0
 always_venv=0