Add ExtraConfig example that always runs on update

Add ExtraConfig example that always runs on update

Some user feedback indicates we need better examples showing how
we can run an extraconfig template, e.g on post-deploy so that it
runs every update, regardless of if a change to the config has
occurred.  We can leverage DeployIdentifier for this, just like with
the puppet deployments.

This can be tested with an example like this:

resource_registry:
  OS::TripleO::NodeExtraConfigPost: tripleo-heat-templates/extraconfig/post_deploy/example_run_on_update.yaml

Change-Id: I45d8f8093ab45c03238ec56651c437128661cb95