os-release: add the DEFAULT_HOSTNAME= setting

os-release: add the DEFAULT_HOSTNAME= setting

The motivation is that variants of the same distro that share the same compiled
rpm want to customize various aspects of the system, in particular the
hostname. In some sense the default hostname is part of the identity of the
system, so setting it through os-release makes sense. In particular, instead of
setting a default value in /etc/hostname, the appropriate default can be baked
into the image, leaving /etc/hostname for local overrides only.

Why make this a separate field instead of e.g. using NAME from os-release?
NAME is already used for other purposes, and it seems likely that people want
to set those independently.