Use nis-domainname.service on all RH platforms
    
    RHEL 8 and Fedora >= 29 use "nis-domainname.service" as service name for
    domainname service. Remove special code in ipaplatform.rhel and for Fedora
    < 28. Only Fedora 29+ is supported by IPA 4.8.
    
    Fixes: https://pagure.io/freeipa/issue/8004
    Signed-off-by: Christian Heimes <cheimes@redhat.com>
    Reviewed-By: Rob Crittenden <rcritten@redhat.com>