Rollback existing nested transacvtion before restarting

SQLAlchemy 1.4 will be adding stricter rules to Connection objects
such that a new transaction cannot be started while an old one hasn't
been cleared.   Ensure this particular test fixture clears
the "savepoint" transaction first before starting a new one.

References: https://github.com/sqlalchemy/sqlalchemy/issues/4712
Change-Id: Idcb616fd3add4d58f22b91865cec8a54fe492093

oslo_db/sqlalchemy/utils.py

@@ -1227,12 +1227,16 @@ class NonCommittingConnectable(object):
 
     """
 
+    _nested_trans = None
+
     def __init__(self, connection):
         self.connection = connection
         self._trans = connection.begin()
         self._restart_nested()
 
     def _restart_nested(self):
+        if self._nested_trans is not None:
+            self._nested_trans.rollback()
         self._nested_trans = self.connection.begin_nested()
 
     def _dispose(self):