Ironic discovery image should report what it is on the console

When the IPA image boots, there is no way of distinguishing it from any other CoreOS image (e.g. the actual booted cluster node) when looking at the console.

The image-customisation-controller should generate ignition that drops a file in /etc/issue.d/ (note that filenames must end with .issue) that tells the user they're looking at an IPA image.

e.g. the agent-based installer does this by overriding /etc/issue: https://github.com/openshift/installer/blob/master/data/data/agent/files/etc/issue.template

We should also set a MOTD for users who ssh in while we are at it, preferably overriding the one that CoreOS sets in /etc/motd that only makes sense in the context of a cluster node (I'm not sure if it's possible to override that one with Ignition though, we might have to settle for dropping it it /etc/motd.d.