iperf3 tests need control over bind address to support tests with NAT'd hosts #1476
Description
Using pscheduler to test throughput against servers running in NAT'd environments, like cloud hosting, requires the test to use the public IP of the instance from the external test participant but use the instance internal (often private) IP from the NAT'd server.
The pscheduler task subcommand and throughput test have some support for specify bind addresses, but these do not get translated to the correct bind behavior when running iperf3.
An easy option is to avoid specifying an iperf3 bind parameter "-B" for command invocations. This causes iperf3 to bind on any interface and allows the test to proceed. Unfortunately, the only why to cause iperf3 to be called in this way is to submit pscheduler thoughput tasks with only a destination host parameter, e.g. pscheduler task throughput -d <destination>. These test specifications work to test throughput to and from a NAT'd host but require that the test is submitted from a shell on source host (so that the source address is implied).
This works because the code is written to not use the -B bind parameter to iperf3 if there is no source host provided in the test specification, in run_client() and run_server().
This doesn't work with throughput tests specify both a source and destination host. In those cases, the -B parameter is passed to the server and the client iperf3 invocations and will default to the public IP of the NAT'd host. This causes the iperf3 bind to fail and kills the command. The pscheduler task result then reports a timeout and the test fails. Specifying the various bind parameters of the task subcommand or throughput test doesn't work either because these do not make it through to the iperf3 command construction in a way that makes sense for the NAT'd use-case.
Since scheduled tests and third-party tests specify both source and destination hosts in the test specification the existing code prevents these tests from working with hosts that are using NAT. This prevents regular test for hosts in cloud environments.