ipatests: mark known failures as xfail
    
    Commit 5dbcc1a9d30cdb0bc1c4f8476be37a3ef781f9be marked
    the base class method test_replica0_with_ca_kra_dns_install
    as known failure, but this does not work with inherited
    classes. The child class methods need to be marked
    themselves as known failures with @pytest.mark.xfail
    
    Furthermore, TestInstallWithCA_KRA2 and TestInstallWithCA_KRA_DNS2
    tests should succeed because the master is installed with KRA
    (issue 7651 is related to replica install with --setup-kra
    when it is the first KRA instance).
    
    Related to https://pagure.io/freeipa/issue/7651
    
    Reviewed-By: Rob Crittenden <rcritten@redhat.com>