Add option defaults for Flink Python tests

I want to run a single Flink Python test:

python -m apache_beam.runners.portability.flink_runner_test FlinkRunnerTest.test_metrics

But I get this error:
TypeError: expected str, bytes or os.PathLike object, not NoneType

Turns out flink_job_server_jar isn't set, and there's no default value. We should set a default.

We should also change the default environment type to LOOPBACK for basic testing purposes because it requires the least setup.