use the marionette_extension pypi package

Switching over to the pypi package for installing the marionette extension. I removed bundles/ and modified our setup.py files to include the new package, updated certsuite/harness.py to call the package, added a webapi_tests/setup_venv.sh script that can be shared between the updated webapi_tests/install_marionette_extension.sh and webapi_tests/setup_and_run.sh scripts.