Add a 'warn' ansible module

This module simply adds the provided string to the `warnings` ansible
output.

Change-Id: Ia0046b284228a5da7257921bc5446266c4c3c0d8

releasenotes/notes/add-warn-helper-a586ba13c7e8b43b.yaml

@@ -0,0 +1,5 @@
+---
+features:
+  - |
+    Add a new `warn` ansible module that simply adds a string to the 'warnings'
+    ansible output.

validations/library/warn.py

@@ -0,0 +1,32 @@
+#!/usr/bin/env python
+# Copyright 2017 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.
+
+from ansible.module_utils.basic import *  # noqa
+
+
+def main():
+    module = AnsibleModule(argument_spec=dict(
+        msg=dict(required=True, type='str'),
+    ))
+
+    msg = module.params.get('msg')
+
+    module.exit_json(changed=False,
+                     warnings=[msg])
+
+
+if __name__ == '__main__':
+    main()