Sync latest install_venv_common from oslo.

This commit syncs the latest install_venv_common from oslo which
fixes an issue with importing from tempest.openstack.common. This
is needed to run tools/install_venv.py in a non-devstack environment.

Change-Id: I37812c9e0cd432a05ef63eba38fb91ea6df98a0b
diff --git a/tools/install_venv_common.py b/tools/install_venv_common.py
index 3dbcafe..8b676bf 100644
--- a/tools/install_venv_common.py
+++ b/tools/install_venv_common.py
@@ -25,6 +25,13 @@
 import subprocess
 import sys
 
+
+possible_topdir = os.getcwd()
+if os.path.exists(os.path.join(possible_topdir, "tempest",
+                               "__init__.py")):
+    sys.path.insert(0, possible_topdir)
+
+
 from tempest.openstack.common import cfg