Replace DbMigrationError with DBMigrationError

because DbMigrationError is deprecated

Change-Id: I961a45d47cd0b0322a2484ed43692887d887a767

ironic/db/sqlalchemy/migration.py

@@ -68,7 +68,7 @@ def create_schema(config=None, engine=None):
     #                schema, it will only add the new tables, but leave
     #                existing as is. So we should avoid of this situation.
     if version(engine=engine) is not None:
-        raise db_exc.DbMigrationError("DB schema is already under version"
+        raise db_exc.DBMigrationError("DB schema is already under version"
                                       " control. Use upgrade() instead")
 
     models.Base.metadata.create_all(engine)

ironic/tests/unit/db/sqlalchemy/test_migrations.py

@@ -664,7 +664,7 @@ class MigrationCheckersMixin(object):
     def test_upgrade_and_create_schema(self):
         with patch_with_engine(self.engine):
             self.migration_api.upgrade('31baaf680d2b')
-            self.assertRaises(db_exc.DbMigrationError,
+            self.assertRaises(db_exc.DBMigrationError,
                               self.migration_api.create_schema)
 
     def test_upgrade_twice(self):