[leapp] Unable to upgrade to RHEL 8 behind a NTLM proxy

Issue

While trying an upgrade behind a proxy with credentials, leapp upgrade fails with the error messages below:

Message: Unable to install RHEL 8 userspace packages.
<--cut-->
            Failed to synchronize cache for repo 'rhel-8-for-x86_64-appstream-rpms', ignoring this repo.
            Failed to synchronize cache for repo 'rhel-8-for-x86_64-baseos-rpms', ignoring this repo.
            Error: Unable to find a match: dnf