In RHUI 1.2, we changed from having a configuration file with the list
of channels to reading them dynamically from the content certificate
(the one that Red Hat provides, or in this scenario, the entitlement
certificate that comes from the master).

That presented a problem. Most of the URLs contain variable
substitutions for things like arch and version. For example:

content/dist/rhel/rhui/server/$releasever/$basearch/rhui/1.1/os

We can't use that URL directly to query CDN. The RHUA needs to use not
only valid values for $releasever and $basearch, but all valid values.
In other words, the RHUA needs to download both i386 and x86_64 for
$basearch.

After some investigation, the best approach is to store a file on CDN
that contains the possible values for each of those two variables. The
RHUA then breaks down the URL and does the lookups.

For instance, when the RHUA encounters $releasever in the URL above, it
chops that part off of the URL and looks for a listing file in the same
directory. That's not just a cute name, it's actually called listing. So
in the above example, it'd look for:

content/dist/rhel/rhui/server/listing

That file is a plain-text file containing values for $releasever that we
should use. It's contents are simply:

5Server

Then we encounter the next variable, $basearch. For each value in the
previous listing file, we do another lookup:

content/dist/rhel/rhui/server/5Server/listing

That one is a bit more interesting. That file contains:

i386
x86_64

Putting all of that together, we came out with two repos to sync:

content/dist/rhel/rhui/server/5Server/i386/
content/dist/rhel/rhui/server/5Server/x86_64/

So, this brings us back to our master-slave solution. It's currently
failing because the content certificates still have variable
substitutions in them but those listing files don't exist on the master CDS.

I'd suggest we just add them manually to the master CDS. The other
approach would be to generate a custom entitlement certificate that
doesn't use variables, but I think adding the listing files should be
easier. Let me know if it's not clear where they go or their contents
and we'll go from there.