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This will then just update out of date packages with pip.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: I96dc6940f536feb810d57b3316e9d712aa750d93
Merge "Add an update option to run_tests.sh"