Update update-cloud-images script to use python3.

Use lf-activate-venv() to install openstack based on python3. Also added
error handling (script was ignoring errors by default). Minor cleanup of
syntax that does not effect functionality. Some improvements made to
comments and improved logging.

Issue: RELENG-2647
Change-Id: I4ddc0036bf178ca7185a00454dfa6f26433a8bab
Signed-off-by: Tim Johnson <tijohnson@linuxfoundation.org>