GCEComputeResourceController creates VM incompatible default Python version

[davidxia]

The GCEComputeResourceController class creates a GCE VM whose default Python version is incompatible with the client source code.

This line

pubsub/load-test-framework/src/main/java/com/google/pubsub/flic/controllers/resource_controllers/GCEComputeResourceController.java

Lines 37 to 38 in 6fbb5fb

	private static final String SOURCE_FAMILY =
	"projects/ubuntu-os-cloud/global/images/ubuntu-1604-xenial-v20171026a"; // Ubuntu 16.04 LTS

creates a GCE VM with Ubuntu 16.04 Xenial. The default Python 3 version is 3.5.2.

This client code

pubsub/load-test-framework/python_src/clients/cps_publisher_task.py

Lines 35 to 39 in 6fbb5fb

	class PublisherSubtaskWorker(SubtaskWorker):
	def __init__(self):
	super().__init__()
	self._flow_controller: FlowController = None
	self._pub_id = int(random.random())

uses variable annotations which aren't supported until Python 3.6 [1].

The SOURCE_FAMILY should probably be changed to use an Ubuntu version whose default Python is >= 3.6 like Focal 20.04.

1: https://www.python.org/dev/peps/pep-0526/