discussion: add support for distro-specific defaults

[yuqi-zhang]

Today we use coreos-installer to create the installer iso for FCOS and RHCOS. But in https://github.com/coreos/coreos-installer/blob/master/coreos-installer we have hard-coded defaults for the DEFAULT_IMAGE_URL and GPG_KEY, which should be distro-specific.

After brief discussion with @dustymabe we decided to, for now, apply a patch to RHCOS's coreos-installer package to use different defaults. In the future it is likely better to put those values in a config that's easily overridden, and different builds can apply different defaults per distro. Opening this issue for further discussion on this topic.