Add example showing how to set root password via cloud-init

Add example showing how to set root password via cloud-init

There have been some requests to enable this, and although it's not
something we should enable by default, this shows how you can use
the generic NodeUserData interface to have cloud-init do it.

To use this you create an environment file like:

resource_registry:
  OS::TripleO::NodeUserData: path/to/userdata_root_password.yaml
parameter_defaults:
  NodeRootPassword: insecure_changeme

Obviously this isn't that secure, and thus isn't intended for
production environments, but it may be useful for debugging and
as a further example showing how to configure things via cloud-init.

Change-Id: If87a1e1dbfaf31b84cc0667c9a60bbd3c757d8cd