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This will then just update out of date packages with pip.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