MINOR: Lower producer throughput in flaky upgrade system test

We see the upgrade test failing from time to time. I looked into it and found that the root cause is basically that the test throughput can be too high for the 0.9 producer to make progress. Eventually it reaches a point where it has a huge backlog of timed out requests in the accumulator which all have to be expired. We see a long run of messages like this in the output:

```
{"exception":"class org.apache.kafka.common.errors.TimeoutException","time_ms":1559907386132,"name":"producer_send_error","topic":"test_topic","message":"Batch Expired","class":"class org.apache.kafka.tools.VerifiableProducer","value":"335160","key":null}
{"exception":"class org.apache.kafka.common.errors.TimeoutException","time_ms":1559907386132,"name":"producer_send_error","topic":"test_topic","message":"Batch Expired","class":"class org.apache.kafka.tools.VerifiableProducer","value":"335163","key":null}
{"exception":"class org.apache.kafka.common.errors.TimeoutException","time_ms":1559907386133,"name":"producer_send_error","topic":"test_topic","message":"Batch Expired","class":"class org.apache.kafka.tools.VerifiableProducer","value":"335166","key":null}
{"exception":"class org.apache.kafka.common.errors.TimeoutException","time_ms":1559907386133,"name":"producer_send_error","topic":"test_topic","message":"Batch Expired","class":"class org.apache.kafka.tools.VerifiableProducer","value":"335169","key":null}
```
This can continue for a long time (I have observed up to 1 min) and prevents the producer from successfully writing any new data. While it is busy expiring the batches, no data is getting delivered to the consumer, which causes it to eventually raise a timeout.
```
kafka.consumer.ConsumerTimeoutException
at kafka.consumer.NewShinyConsumer.receive(BaseConsumer.scala:50)
at kafka.tools.ConsoleConsumer$.process(ConsoleConsumer.scala:109)
at kafka.tools.ConsoleConsumer$.run(ConsoleConsumer.scala:69)
at kafka.tools.ConsoleConsumer$.main(ConsoleConsumer.scala:47)
at kafka.tools.ConsoleConsumer.main(ConsoleConsumer.scala)
```
The fix here is to reduce the throughput, which seems reasonable since the purpose of the test is to verify the upgrade, which does not demand heavy load. Note that I investigated several failing instances of this test going back to 1.0 and saw a similar pattern, so there does not appear to be a regression.

Author: Jason Gustafson <jason@confluent.io>

Reviewers: Gwen Shapira

Closes #6907 from hachikuji/lower-throughput-for-upgrade-test

tests/kafkatest/tests/core/upgrade_test.py

@@ -36,7 +36,7 @@ class TestUpgrade(ProduceConsumeValidateTest):
         self.zk.start()
 
         # Producer and consumer
-        self.producer_throughput = 10000
+        self.producer_throughput = 1000
         self.num_producers = 1
         self.num_consumers = 1