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This will then just update out of date packages with pip.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: Iac065f71a64a6a7f7f2ee000ee01be9f78af6754
Merge "Add an update option to run_tests.sh"