Update run-tests.sh to use python 3
run-tests.sh uses python in other places besides running the sdk/python tests (e.g. starting services, etc). Update these instances to use python 3. Specifically, change VENVDIR/bin/activate to VENV3DIR/bin/activate.
If py2 isn't installed, run-tests.sh should succeed (by skipping the py2 tests). If py2 is installed, run-tests.sh should not skip the py2 tests.