make --protocol_version command line argument optional

	modified:   src/bruce/bruce.test.cc
	modified:   src/bruce/config.cc
	modified:   src/bruce/input_dg/any_partition/v0/v0_input_dg.test.cc
	modified:   src/bruce/input_dg/old_v0_input_dg.test.cc
	modified:   src/bruce/input_dg/partition_key/v0/v0_input_dg.test.cc
	modified:   src/bruce/input_thread.test.cc
	modified:   src/bruce/mock_kafka_server/config.cc
	modified:   src/bruce/test_util/mock_kafka_config.cc