script.py.mako: added license header and missing branch_labels

The license header is needed so that new revision files pass pep8
checks.

Change-Id: I42101900ec0f160461f0d1705403865f903cb4ff
Partially-Implements: blueprint online-schema-migrations

neutron_vpnaas/db/migration/alembic_migrations/script.py.mako

@@ -1,3 +1,18 @@
+# Copyright ${create_date.year} <PUT YOUR NAME/COMPANY HERE>
+#
+#    Licensed under the Apache License, Version 2.0 (the "License"); you may
+#    not use this file except in compliance with the License. You may obtain
+#    a copy of the License at
+#
+#         http://www.apache.org/licenses/LICENSE-2.0
+#
+#    Unless required by applicable law or agreed to in writing, software
+#    distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+#    WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
+#    License for the specific language governing permissions and limitations
+#    under the License.
+#
+
 """${message}
 
 Revision ID: ${up_revision}
@@ -9,6 +24,9 @@ Create Date: ${create_date}
 # revision identifiers, used by Alembic.
 revision = ${repr(up_revision)}
 down_revision = ${repr(down_revision)}
+% if branch_labels:
+branch_labels = ${repr(branch_labels)}
+%endif
 
 from alembic import op
 import sqlalchemy as sa