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/cmd/account_generator.py b/tempest/cmd/account_generator.py
index 9e98d90..d8b4885 100755
--- a/tempest/cmd/account_generator.py
+++ b/tempest/cmd/account_generator.py
@@ -480,8 +480,8 @@
 def main(opts=None):
     setup_logging()
     if not opts:
-        LOG.warn("Use of: 'tempest-account-generator' is deprecated, "
-                 "please use: 'tempest account-generator'")
+        LOG.warning("Use of: 'tempest-account-generator' is deprecated, "
+                    "please use: 'tempest account-generator'")
         opts = get_options()
     if opts.config_file:
         config.CONF.set_config_path(opts.config_file)
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)