Add placeholder migrations to allow backports

Inspired by the nova blueprint backportable-db-migrations,
we reserve a range of migration slots to allow for idempotent
and standalone migrations to be backported to stable/grizzly
during the Havana release cycle.

Nova have reserved 10 slots and the historical ratio of
nova:glance migrations is circa 8:1. Hence the thought that
a total of 3 placeholder slots should more than suffice for
the Havana release cycle.

Change-Id: I576be7520f960e4cb4dcc6928035595fab7e459f

glance/db/sqlalchemy/migrate_repo/versions/023_placeholder.py

@@ -0,0 +1,25 @@
+# vim: tabstop=4 shiftwidth=4 softtabstop=4
+
+# Copyright 2013 OpenStack Foundation
+# Copyright 2013 Red Hat, Inc.
+# All Rights Reserved.
+#
+#    Licensed under the Apache License, Version 2.0 (the "License"); you may
+#    not use this file except in compliance with the License. You may obtain
+#    a copy of the License at
+#
+#         http://www.apache.org/licenses/LICENSE-2.0
+#
+#    Unless required by applicable law or agreed to in writing, software
+#    distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+#    WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
+#    License for the specific language governing permissions and limitations
+#    under the License.
+
+
+def upgrade(migrate_engine):
+    pass
+
+
+def downgrade(migration_engine):
+    pass

glance/db/sqlalchemy/migrate_repo/versions/024_placeholder.py

@@ -0,0 +1,25 @@
+# vim: tabstop=4 shiftwidth=4 softtabstop=4
+
+# Copyright 2013 OpenStack Foundation
+# Copyright 2013 Red Hat, Inc.
+# All Rights Reserved.
+#
+#    Licensed under the Apache License, Version 2.0 (the "License"); you may
+#    not use this file except in compliance with the License. You may obtain
+#    a copy of the License at
+#
+#         http://www.apache.org/licenses/LICENSE-2.0
+#
+#    Unless required by applicable law or agreed to in writing, software
+#    distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+#    WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
+#    License for the specific language governing permissions and limitations
+#    under the License.
+
+
+def upgrade(migrate_engine):
+    pass
+
+
+def downgrade(migration_engine):
+    pass

glance/db/sqlalchemy/migrate_repo/versions/025_placeholder.py

@@ -0,0 +1,25 @@
+# vim: tabstop=4 shiftwidth=4 softtabstop=4
+
+# Copyright 2013 OpenStack Foundation
+# Copyright 2013 Red Hat, Inc.
+# All Rights Reserved.
+#
+#    Licensed under the Apache License, Version 2.0 (the "License"); you may
+#    not use this file except in compliance with the License. You may obtain
+#    a copy of the License at
+#
+#         http://www.apache.org/licenses/LICENSE-2.0
+#
+#    Unless required by applicable law or agreed to in writing, software
+#    distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+#    WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
+#    License for the specific language governing permissions and limitations
+#    under the License.
+
+
+def upgrade(migrate_engine):
+    pass
+
+
+def downgrade(migration_engine):
+    pass