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This will then just update out of date packages with pip.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: I49a1cbcb62e7f429eca3efe949b8aa90507723c4
Merge "Add an update option to run_tests.sh"