Add stream metadata for RHCOS

This implements part of the plan from:
https://github.com/openshift/os/issues/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 exposed via a new
`openshift-install coreos print-stream-json` command.

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

HOWEVER as a (really hopefully temporary) hack, we *duplicate*
the metadata so that IPI installs use the new stream format,
and UPI CI jobs can still use the old format (with different RHCOS versions).

We will port the UPI docs and CI jobs after this merges.

Co-authored-by: Matthew Staebler <staebler@redhat.com>

docs/user/aws/install_upi.md

@@ -21,8 +21,8 @@ $ openshift-install create install-config
 ### Optional: Create Encrypted AMIs
 
 The IPI-based installer creates an encrypted AMI by default. If you wish to have an encrypted AMI for UPI-based
-installs, you will need to create it directly. You can find a list of the appropriate base AMIs 
-[here](../../../data/data/rhcos.json).
+installs, you will need to create it directly.  See [CoreOS bootimages](../overview.md#coreos-bootimages) for more information
+about bootimages, including how to find the AMI identifiers.
 
 You will make an encrypted copy of the AMI according to the [AWS documentation][encrypted-copy].