Deny to migrate to lower schema version

Deny migrating to lower version and remove downgrades testing
relying on
http://specs.openstack.org/openstack/openstack-specs/specs/no-downward-sql-migration.html

Change-Id: I72421259deb9f28501b9d5ca4b9ccc43a8da7831

heat/db/sqlalchemy/api.py

@@ -1035,6 +1035,9 @@ def sync_point_update_input_data(context, entity_id,
 
 def db_sync(engine, version=None):
     """Migrate the database to `version` or the most recent version."""
+    if version is not None and int(version) < db_version(engine):
+        raise exception.Error(_("Cannot migrate to lower schema version."))
+
     return migration.db_sync(engine, version=version)
 
 

heat/tests/db/test_migrations.py

@@ -42,8 +42,8 @@ class HeatMigrationsCheckers(test_migrations.WalkVersionsMixin,
                              common.FakeLogMixin):
     """Test sqlalchemy-migrate migrations."""
 
-    snake_walk = True
-    downgrade = True
+    snake_walk = False
+    downgrade = False
 
     @property
     def INIT_VERSION(self):