Using non-versioned scripts for `build_file` means every Python version is
tested (under Tox), and this is multiplied by each Python version running Tox.

For example, when the `python38` config is used to run Tox, the
Tox layer runs on Python 3.8. This then tests all of the Python versions
individually, including, for example, python27 tests.

This change fixes the `build_file` paths to point to the same-pathed build
script. For example, `kokoro/linux/python27/presubmit.cfg` now uses
`kokoro/linux/python27/build.sh`.

Some additional fixes:

*  Use `python -m tox` in tests.sh instead of just `tox`. This helps non-site
   installations of tox, where the `tox` script may not be on `$PATH`.
*  Ensure tox (and other Python build-related packages) are available in
   Python testing images. (New images have been pushed.)
*  Disable `--warnings_as_errors` due to deprecated function.
*  Remove apt lists per [Docker best practices][1].

[1]: https://docs.docker.com/develop/develop-images/dockerfile_best-practices/