In "test_restart_rpc_on_sighup_multiple_workers", the test needs to
wait until the RPC workers have been properly launched by
``oslo_service.service.ProcessLauncher.launch_service``. Once this
method returns, it is guaranteed that the child worker processes
are running and the signal process handlers are attending the
SIGHUP signal that will reset them.
Closes-Bug: #1938428
Change-Id: I1dc56092d099223accc3aefa8e303310c4f6787e