When `get_fallback_rhsm_secrets` was used, `Subscriptions.repositories`
was None, and `get_secrets` never returned the fallback secrets.
So check if `repositories` is None before
iterating over it, otherwise return the fallback secrets.
The problem is that some deployments might not have the redhat.repo
file, yet they might have the key and certificate to access Red Hat CDN.
If that was the case, the new approach would cause a regression compared
to the previous behavior.
This patch uses the previous method if the redhat.repo file is not
found or does not contain any matching URL.
The previous version covered too few use cases, more specifically a
single subscription. That is of course not the case for many hosts, so
osbuild needs to understand subscriptions.
When running org.osbuild.curl source, read the
/etc/yum.repos.d/redhat.repo file and load the system subscriptions from
there. While processing each url, guess which subscription is tied to
the url and use the CA certificate, client certificate, and client key
associated with this subscription. It must be done this way because the
depsolving and fetching of RPMs may be performed on different hosts and
the subscription credentials are different in such case.
More detailed description of why this approach was chosen is available
in osbuild-composer git: https://github.com/osbuild/osbuild-composer/pull/1405
