Fixes #28698 - route docker registry through apache

[jlsherrill]

for pulp3

This change changes the expectation of the content_app_url setting
provided by the pulp3 smart proxy plugin. Now katello will be
proxying through apache to a special endpoint /pulpcore_registry/
that requires a client cert with a CN of 'admin'.

[theforeman-bot]

Issues: #28698

[jlsherrill]

To Test:

1. redeploy my forklift pr here: Add a test role for Pulp3 theforeman/forklift#980
   This will add the following apache config:

<Location /pulpcore_registry/v2/>
   SSLRequire %{SSL_CLIENT_S_DN_CN} eq "adminz"
</Location>

ProxyPass /pulpcore_registry/v2/ http://127.0.0.1:24816/v2/
ProxyPassReverse /pulpcore_registry/v2 http://127.0.0.1:24816/v2

and will change your content_app_url in the smart proxy plugin config

2. refresh the smart proxy
3. restart httpd
4. create and sync a docker repository
5. find the docker pull url and pull it with podman (without the :5000)

podman login devel.balmora.example.com
podman pull devel.balmora.example.com/default_organization-docker_test-docker

[ianballou]

I was able to do podman login and podman search to find my container image but podman pull failed and the server had these errors: https://gist.github.com/ianballou/15794034276e7f54da13d1340e7db968

Edit: Never mind this, it worked just fine. Reviewing the code now!

[jlsherrill]

[test katello]