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This will then just update out of date packages with pip.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: I7da0226a6621ed7120b00eafd8d0ce75c1c2a67b
Merge "Add an update option to run_tests.sh"