Add license header to migrations template

Change-Id: I0b783dc67d89cbf15fe61bf7097aa2749005e0a9

manila/db/migrations/alembic/script.py.mako

@@ -1,3 +1,15 @@
+# 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}