Support conditional user creation

[cgwalters]

Splitting this out of discussion here:

coreos/afterburn#90 (comment)

Today CL precreates a core user. Not all distributions will want to do that; I personally prefer the cloud-init semantic where it has a default user that's created dynamically if and only if you do not specify a different username.

I suspect for most people who are defining a different user with Ignition than core for CL, the core user is unused. And it could be a source of conflict if the machine is joined to a LDAP domain or whatever that happens to have a core user.

[ajeddeloh]

I don't really like the idea of conditional anything in Ignition; it's a declarative spec. We're not perfect at it, but it's what we aim for. What we can do is something akin to (and simpler than) the partition reuse semantics. This would enable a config to say core should not exist (and whether it should be deleted or Ignition should fail if it does) and that another user should exist.

Thoughts?

[bgilbert]

My understanding of the proposal is that Ignition would create the core user only if no other users are specified in the config. So the config would still be self-contained, just weirdly implicit.

But yeah, adding the ability to delete users does seem like a nice solution.

[ajeddeloh]

Given how we changed fcos to create core via the Ignition base config, Is this now part of #738? @cgwalters

[cgwalters]

Yeah, fine to close this in favor of #738 from my PoV.