subprocess.Popen hangs after use of gRPC channel

subprocess.Popen randomly hangs after use of gRPC channel. This makes cross-language wordcount test fail because the test uses Popen to launch Dockerized Flink job server in `PortableRunner.run_pipeline` after use of gRPC channel for the expansion service in `ExternalTransform.expand`. Few symptoms are listed below:

• Hanging at `docker_path = check_output(['which', 'docker']).strip()`
• Hanging at `self.docker_process = Popen(cmd)`
• Crashing with `assertion failed: pthread_mutex_lock(mu) == 0` message