Update mirror registry to use `hostname` instead of reading `/etc/hosts`

See upstream issue: https://github.com/quay/mirror-registry/issues/60
RH Quay Space: https://chat.google.com/room/AAAAEfCP9z8/Gt_7kagLgVo

getFQDN() relies on the fqdn library to determine fqdn. This library tries to read /etc/hosts directly. On some RHEL installs with uncommon DNS configurations this fails and install aborts. We should update this function to instead use `hostname -f` to pull the fqdn.