Use stream metadata for RHCOS, bump x86_64 to 48.83.202102230316-0

This implements part of the plan from:
openshift/os#477

When we originally added the pinned RHCOS metadata `rhcos.json`
to the installer, we also changed the coreos-assembler `meta.json`
format into an arbitrary new format in the name of some cleanups.
In retrospect, this was a big mistake because we now have two
formats.

Then Fedora CoreOS appeared and added streams JSON as a public API.

We decided to unify on streams metadata; there's now a published
Go library for it: https://github.com/coreos/stream-metadata-go

Among other benefits, it is a single file that supports multiple
architectures.

UPI installs should now use stream metadata, particularly
to find public cloud images.

This is an important preparatory step for exposing this via
`oc` as well as having something in the cluster update to
it.