Make storyboard idempotent

Previously the curl command was itself idempotent but this
was transparent to Puppet, meaning the node reported as
always changed. I have, somewhat akwardly, taught Puppet how to
check for changed files on the remote server. It pulls only HTTP
headers to save time/bandwidth.

Change-Id: I539e67f0c82ad91450dd3720512b92518e70c122