Don't call 'begin()' on existing transaction

Resolve the following RemovedIn20Warning warning:

  Calling .begin() when a transaction is already begun, creating a 'sub'
  transaction, is deprecated and will be removed in 2.0.  See the
  documentation section 'Migrating from the nesting pattern' for
  background on how to migrate from this pattern.

Change-Id: I59e45dfea8fcbf72fc6e34345e510554cbdd138e
Signed-off-by: Stephen Finucane <stephenfin@redhat.com>

oslo_db/tests/fixtures.py

@@ -45,11 +45,6 @@ class WarningsFixture(fixtures.Fixture):
             message=r'The Session.autocommit parameter is deprecated .*',
             category=sqla_exc.SADeprecationWarning)
 
-        warnings.filterwarnings(
-            'once',
-            message=r'Calling \.begin\(\) when a transaction is already .*',
-            category=sqla_exc.SADeprecationWarning)
-
         # ...plus things that aren't our fault
 
         # FIXME(stephenfin): These are caused by sqlalchemy-migrate, not us,

oslo_db/tests/sqlalchemy/test_utils.py

@@ -700,8 +700,7 @@ class TestMigrationUtils(db_test_base._DbTestCase):
 
         test_table.create(engine)
         with engine.connect() as conn, conn.begin():
-            with conn.begin():
-                conn.execute(test_table.insert(), values)
+            conn.execute(test_table.insert(), values)
         return test_table, values
 
     def test_drop_old_duplicate_entries_from_table(self):