Python 3 deprecated the logger.warn method in favor of warning

Python 3 deprecated the logger.warn method, see:
https://docs.python.org/3/library/logging.html#logging.warning, so we
prefer to use warning to avoid DeprecationWarning.

Change-Id: I873e4d8da3bda87041f4041371d0dbdae541b92e
diff --git a/tempest/stress/stressaction.py b/tempest/stress/stressaction.py
index c8bd652..cf0a08a 100644
--- a/tempest/stress/stressaction.py
+++ b/tempest/stress/stressaction.py
@@ -85,8 +85,8 @@
             finally:
                 shared_statistic['runs'] += 1
                 if self.stop_on_error and (shared_statistic['fails'] > 1):
-                    self.logger.warn("Stop process due to"
-                                     "\"stop-on-error\" argument")
+                    self.logger.warning("Stop process due to"
+                                        "\"stop-on-error\" argument")
                     self.tearDown()
                     sys.exit(1)