Revisit identifiers spec

Partially addresses #1135.

This is the result of discussions in the past few days about imprecision and confusion.  Hopefully simpler.  There are a few FIXME comments in the doc - discuss.
We need to support pods from config files, and I think it is not reasonable to expect every machine to have a different config file.  Unless we make configs be scripts, and save the result.  Blech.