ipatests: re-add test_dnssec.py::TestInstallDNSSECFirst in gating
    
    The test was temporarily removed because of a known issue
    but the issue is now fixed.
    
    Related: https://pagure.io/freeipa/issue/8496
    Signed-off-by: Florence Blanc-Renaud <flo@redhat.com>
    Reviewed-By: Rob Crittenden <rcritten@redhat.com>
    Reviewed-By: Sergey Orlov <sorlov@redhat.com>
    Reviewed-By: Alexander Bokovoy <abokovoy@redhat.com>