Allow tools/install_venv_common.py to be run from within the source directory.

(Copy latest update from oslo)

Not doing this raised exceptions when it tried importing modules
that didn't exist in the path.

Fixes LP# 1112484

Change-Id: I2be7bde0bf873a22318617b987a76b209caef22a
Merge "Allow tools/install_venv_common.py to be run from within the source directory."