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.
I don't want to assume that files on a minion are coordinated.  In this case the blob should be a hash of hostname and filename, or something like that.  The same config file on two minions should produce different hashes.