Update the `python-multi` docker image to use `python-base` as the base image

[ohmayr]

Use https://github.com/googleapis/testing-infra-docker/blob/83f95e41817f986408bca52ce39352a6b6923e4f/python/cloud-devrel-kokoro-resources/python-base as the base image for python-multi.

This needs to be done for all python images that we maintain, where possible.