Boolean check on whether a Pi "should" be provisioned

[jjjcarels]

Not really an "issue" as much as a request... If there could be a check whether a known Pi "should" be reprovisioned when it netboots (allowing a bootflow where it netboots, finds it shouldn't be reprovisioned an image, and continues normally), the cmprovision would be a wonderful basis for a cluster bootstrap.

Build your golden-image, use the pre/post scripts to pull in updates, and (re-)join the cluster.

[jayjayseal]

See: #48