build: use trampoline_v2 for python samples and allow custom dockerfile (#283)

Source-Link: https://github.com/googleapis/synthtool/commit/a7ed11ec0863c422ba2e73aafa75eab22c32b33d
Post-Processor: gcr.io/cloud-devrel-public-resources/owlbot-python:latest@sha256:6e7328583be8edd3ba8f35311c76a1ecbc823010279ccb6ab46b7a76e25eafcc

Co-authored-by: Owl Bot <gcf-owl-bot[bot]@users.noreply.github.com>
13 files changed