Filter out centos8 from execute OS list

Filtered out centos8 from supported execute OS list

templates/htcondor.txt

@@ -171,7 +171,7 @@ Order = 20
         Label = Execute Base OS
         ParameterType = Cloud.Image
         DefaultValue = cycle.image.centos7
-        Config.Filter := !(Package in {"cycle.image.win2016", "cycle.image.ubuntu18", "cycle.image.ubuntu20"})
+        Config.Filter := !(Package in {"cycle.image.win2016", "cycle.image.ubuntu18", "cycle.image.ubuntu20", "cycle.image.centos8"})
 
         [[[parameter SchedulerClusterInitSpecs]]]
         Label = Scheduler Cluster-Init