[CI] Change to specifying konnectivity-agent autoscaler configmap

097cf518523a2fce283928be79b68f849d718bfb attempted to prevent konnectivity-agent
autoscaling by disabling the autoscaler (by setting its replica count to zero) and
explicitly setting the replica count of the konnectivity-agent deployment. It
turns out the GKE add-on manager will automatically reconcile the replica count
of the horizontal autoscaler, so this is not a viable option. The configmap
only is ensured to exist by the add-on manager, so switch to adjusting that.