resolve question marks in /etc/hostname to characters hashed from machine ID (#36647)

So I have a bunch of particle os instances around, that I frequently
factory reset. and it's confusing, since they all have the same name.
Let's do something about this, and extend the hostname setup logic a bit
to deal better with "cattle" rather than "pet" deployments.
Specifically: if a hostname in /etc/hostname contains a bunch of
question marks we'll replace it with hex chars hashed from the machine
id.

This allows us to do something like this:

hostnamectl set-hostname --static 'funky-????-????-???'

and we'll end up with a hostname like `funky-baf4-b653-e230`

diff --cc TODO
Simple merge