Use install_venv_common.py from oslo.

This syncs install_venv_common.py from oslo and reworks the
tools/install_venv.py script to use the new library.

Change-Id: I47b1526c2c81cc1e6b3223dabbf6b5ad674252a7
Add an update option to run_tests.sh

This commit adds an update option to run_tests.sh. This option
is used to rerun install_venv.py on an already installed venv.
This will then just update out of date packages with pip.

Change-Id: I808f019de688387b14bc7a43eea4e45b05948e6b
Merge "Use install_venv_common.py from oslo."
Merge "Add an update option to run_tests.sh"