Simplify noxfile setup.

[kurtisvg]

This PR simplifies the noxfile setup, causing each project use noxfile.py in the project folder. If no noxfile is found, it will search for a noxfile-template.py in the nearest parent folder and use that instead. This will allow per-project configuration of sample versions (and other tests).

Other things:

• Tests python 3.6 and 3.7 by default
• Adds Python 3.8 kokoro build configs (won't be active until job is also updated)
• Removes the shared testing/requirements.py and added project specific requirements-tests.py instead
• Configures appengine/standard to only test Python 2.7