Don't call mapper() outside of declarative registry

Resolve the following RemovedIn20Warning:

  Calling the mapper() function directly outside of a declarative
  registry is deprecated. Please use the
  sqlalchemy.orm.registry.map_imperatively() function for a classical
  mapping.

Change-Id: I92a7ccdd48eedd4c788384033743daf50a9dc113
Signed-off-by: Stephen Finucane <stephenfin@redhat.com>

oslo_db/tests/fixtures.py

@@ -52,11 +52,6 @@ class WarningsFixture(fixtures.Fixture):
             message=r'The Engine.execute\(\) method is considered legacy .*',
             category=sqla_exc.SADeprecationWarning)
 
-        warnings.filterwarnings(
-            'once',
-            message=r'Calling the mapper\(\) function directly outside .*',
-            category=sqla_exc.SADeprecationWarning)
-
         warnings.filterwarnings(
             'once',
             message=r'The current statement is being autocommitted .*',

oslo_db/tests/sqlalchemy/test_enginefacade.py

@@ -24,7 +24,7 @@ from oslo_context import context as oslo_context
 from sqlalchemy import Column
 from sqlalchemy import Integer
 from sqlalchemy import MetaData
-from sqlalchemy.orm import mapper
+from sqlalchemy.orm import registry
 from sqlalchemy.orm import Session
 from sqlalchemy import select
 from sqlalchemy import String
@@ -1671,11 +1671,13 @@ class LiveFacadeTest(db_test_base._DbTestCase):
         metadata.create_all(self.engine)
         self.addCleanup(metadata.drop_all, self.engine)
 
+        reg = registry()
+
         class User(object):
             def __init__(self, name):
                 self.name = name
 
-        mapper(User, user_table)
+        reg.map_imperatively(User, user_table)
         self.User = User
 
     def _assert_ctx_connection(self, context, connection):

oslo_db/tests/sqlalchemy/test_exc_filters.py

@@ -23,7 +23,7 @@ from sqlalchemy.engine import url as sqla_url
 from sqlalchemy import event
 import sqlalchemy.exc
 from sqlalchemy.orm import declarative_base
-from sqlalchemy.orm import mapper
+from sqlalchemy.orm import registry
 from sqlalchemy import sql
 
 from oslo_db import exception
@@ -1046,10 +1046,13 @@ class IntegrationTest(db_test_base._DbTestCase):
         self.test_table.create(self.engine)
         self.addCleanup(self.test_table.drop, self.engine)
 
+        reg = registry()
+
         class Foo(object):
             def __init__(self, counter):
                 self.counter = counter
-        mapper(Foo, self.test_table)
+
+        reg.map_imperatively(Foo, self.test_table)
         self.Foo = Foo
 
     def test_flush_wrapper_duplicate_entry(self):