Fixes #2.

• Adds the standard requirements.txt file to each quickstart directory.
  • Rather than "pip install ", we are using the standard "pip install -r".
  • This change is independent of DevSite instructions.

This ensures the compatibility of the dependencies so that we don't see a quickstart randomly break.

Tested locally.

Ideally the user has venv set up, but we've never enforced that or added that to the quickstart docs.

This requirements.txt setup is similar to what we can see in the GCP python repo.