Replace 'raise StopIteration' with 'return'

With PEP 479, the behaviour of StopIteration is changing. Raising it to
stop a generator is considered incorrect and from Python 3.7 this will
cause a RuntimeError. The PEP recommends using the return statement.

More details: https://www.python.org/dev/peps/pep-0479/#examples-of-breakage

Change-Id: I7b35bc0525b599950a7fa0e58caeffcedb6bd18b

tripleoclient/workflows/base.py

@@ -71,7 +71,7 @@ def wait_for_messages(mistral, websocket, execution, timeout=None):
             # Workflows should end with SUCCESS or ERROR statuses.
             if payload.get('status', 'RUNNING') != "RUNNING" or \
                     mistral.executions.get(execution.id).state != "RUNNING":
-                raise StopIteration
+                return
     except exceptions.WebSocketTimeout:
         check_execution_status(mistral, execution.id)
         raise