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This will then just update out of date packages with pip.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
Add migration 18 - create the image_locations table

This migration simply creates the new image_locations table. It does
not yet use the table for anything.

Related to bp multiple-image-locations

Change-Id: I1536c9276bc807aac26849e37556b465e8b5c9eb
Merge "Use install_venv_common.py from oslo."
Merge "Add an update option to run_tests.sh"