This is another of the timeout_on_sleeping_server bugs. I didn't spot one from cloud_to_prod on python, so creating a new issue. These can be merged into one if the root cause is the same.

2019-08-01 16:49:59,634 TIMEOUT: cloud_to_prod:python:default:timeout_on_sleeping_server:tls [pid=29682, time=187.8sec]
2019-08-01 16:49:59,923 FAILED: Some tests failed

https://source.cloud.google.com/results/invocations/5b6c5b07-dba8-4d23-9805-14702d2c3fa5/targets/grpc%2Fcore%2Fpull_request%2Flinux%2Fgrpc_interop_toprod/log

seen on #19316