Add openshift-install ignition convert

[cgwalters]

The transition to Ignition Spec 3 with 4.6 creates a
discontinuity. Some users want to update their bootimages,
e.g. for a cluster originally provisioned as 4.4 but upgraded
in place to 4.6, it should be possible to directly use RHCOS 4.6
bootimages for new workers.

In some cases in fact, this could be required for things like
adding a node with newer hardware.

The main stumbling block here is the pointer ignition config.
Since openshift-install already includes Ignition bits, let's
add translation capability here using
https://github.com/coreos/ign-converter
the same as the MCO uses.

xref openshift/enhancements#492 (comment)
xref https://bugzilla.redhat.com/show_bug.cgi?id=1884750

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[cgwalters]

We've hit this when porting some of our UPI tests as well as the proxy test - that one we ported to FCOS, but if we'd had this PR we could have kept the spec2 configs for a while and used this to upconvert.

[staebler]

My gut reaction to this is that it is not an appropriate command for openshift-install. The purview of openshift-install is installing clusters and destroying clusters: It is not supporting day-2 administration of clusters. As such, openshift-install is not a good home for tooling to support adding new nodes to an existing cluster. Although I do not like the idea of adding more to its hodgepodge of commands, I feel that oc is stronger candidate for this command than openshift-install.

[cgwalters]

It is not supporting day-2 administration of clusters.

Agree. However, today we have a model where admins often need to download openshift-install and the RHCOS bootimages at the same time, so they will have both. Plus openshift-install generated these pointer configs, so it's somewhat natural to have it support converting them.

Plus openshift-install already vendors Ignition code.

I feel that oc is stronger candidate for this command than openshift-install.

Yeah, that was my original proposal as well...I dunno. I flip-flopped based on the above but yeah I am OK with doing oc.

[cgwalters]

OK moved to openshift/oc#628