Add Apache 2.0 license to source file

As per OpenStack licensing guide lines [1]:
[H102 H103] Newly contributed Source Code should be licensed under
the Apache 2.0 license.
[H104] Files with no code shouldn't contain any license header nor
comments, and must be left completely empty.

[1] http://docs.openstack.org/developer/hacking/#openstack-licensing

Change-Id: I74d25d88ac285c456cbb8874deb000ed782c2192

tools/xenserver/destroy_cached_images.py

@@ -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.
+
 """
 destroy_cached_images.py
 

tools/xenserver/stress_test.py

@@ -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.
+
 """
 This script concurrently builds and migrates instances. This can be useful when
 troubleshooting race-conditions in virt-layer code.