build: use trampoline_v2 for python samples and allow custom dockerfile

[busunkim96]

• Use trampoline_v2 (Docker CI Helper) for all python samples.

• Optionally allow samples to run with a Dockerfile specified in the library repository at .kokoro/docker/samples. On a successful test run, this Dockerfile is uploaded to gcr.io/cloud-devrel-kokoro-resources/{repo-name}-samples-docker. The option is enabled by passing custom_samples_dockerfile=True to common.py_library in owlbot.py.

   templated_files = common.py_library(microgenerator=True, custom_samples_dockerfile=True)

See googleapis/python-asset#308 for an example of moving samples to to trampoline_v2.

See googleapis/python-recaptcha-enterprise#128 for an example of moving to trampoline_v2 + using a custom Dockerfile.