Allow tools/install_venv_common.py to be run from within the source directory.
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# 1116269

Change-Id: I3e2ed3ac96b51ec89def911054720d11bbcd9407

wolverineav/networking-bigswitch,stackforge/networking-bigswitch,openstack/networking-bigswitch,wolverineav/networking-bigswitch,openstack/networking-bigswitch,stackforge/networking-bigswitch