RHEL7: "leapp preupgrade" が失敗し、"Unable to use yum successfully" というメッセージが表示される

Issue

• "leapp preupgrade" または "leapp upgrade" を実行すると、次のメッセージが表示されます。

  [ERROR] Actor: target_userspace_creator
  Message: Unable to use yum successfully
  Summary:
      Details: Command ['systemd-nspawn', '--register=no', '--quiet', '-D', '/var/lib/leapp/scratch/mounts/root_/system_overlay', '--bind=/etc/hosts:/etc/hosts', '--setenv=..., 'yum', 'clean', 'all'] failed with exit code 1.
      Stderr: Host and machine ids are equal (XXX): refusing to link journals
               There are no enabled repos.
               Run "yum repolist all" to see the repos you have.
               To enable Red Hat Subscription Management repositories:
                   subscription-manager repos --enable <repo>
               To enable custom repositories:
                   yum-config-manager --enable <repo>