Lower severity of log trace for DB integrity error

LOG.exception is not appropriate, since a concurrent
port creation can occur and it has been dealt with
in the except block.

This avoids spooking users who are on the hunt for
real bugs.

Supports fix for 1239637

Change-Id: I9489702ee63db0b004fe41195fa4727af261d514

neutron/plugins/ml2/plugin.py

@@ -425,7 +425,7 @@ class Ml2Plugin(db_base_plugin_v2.NeutronDbPluginV2,
             except os_db.exception.DBError as e:
                 if isinstance(e.inner_exception, sql_exc.IntegrityError):
                     msg = _("A concurrent port creation has occurred")
-                    LOG.exception(msg)
+                    LOG.warning(msg)
                     continue
                 else:
                     raise