downsize arcus control node to try to avoid CI failures

Author: sjpb

environments/.stackhpc/terraform/ARCUS.tfvars

@@ -1,5 +1,5 @@
 cluster_net = "portal-internal"
 cluster_subnet = "portal-internal"
 vnic_type = "normal"
-control_node_flavor = "vm.ska.cpu.general.quarter"
+control_node_flavor = "vm.ska.cpu.general.eighth"
 other_node_flavor = "vm.ska.cpu.general.small"