Fix named.conf update bug NAMED_DNSSEC_VALIDATION
    
    Commit a5cbdb57e50cfc62f61affda19ce878b2abd33de introduced a bug when
    updating IPA from 4.8.6 to 4.8.7. NAMED_DNSSEC_VALIDATION template
    variable was not declared.
    
    Fixes: https://pagure.io/freeipa/issue/8363
    Signed-off-by: Christian Heimes <cheimes@redhat.com>
    Reviewed-By: Alexander Bokovoy <abokovoy@redhat.com>