Description of problem:
Failed to delete machineautoscaler if related machineset doesn't exist.

Version-Release number of selected component (if applicable):
clusterversion: 4.0.0-0.alpha-2019-01-24-003716

How reproducible:
Always

Steps to Reproduce:
1. Create a machineautoscaler which targets a node group zhsun-worker-us-east-2c
apiVersion: autoscaling.openshift.io/v1alpha1
kind: MachineAutoscaler
metadata:
  finalizers:
  - machinetarget.autoscaling.openshift.io
  name: autoscale-us-east-2c
  namespace: openshift-cluster-api
spec:
  maxReplicas: 3
  minReplicas: 1
  scaleTargetRef:
    apiVersion: cluster.k8s.io/v1alpha1
    kind: MachineSet
    name: worker-us-east-2a
status: {}
2. Delete machineset zhsun-worker-us-east-2c
3. Delete machineautoscaler
4. Check logs

Actual results:
Machineautoscaler couldn't be deleted.

$ oc get machineautoscaler
NAME                     REF KIND     REF NAME                  MIN       MAX       AGE
autoscale-us-east-2a     MachineSet   zhsun-worker-us-east-2a   1         2         1h
autoscale-us-east-2b     MachineSet   zhsun-worker-us-east-2b   1         2         1h
autoscale-us-east-2c     MachineSet   zhsun-worker-us-east-2c   1         2         3h

$ oc get machineset
NAME                      DESIRED   CURRENT   READY     AVAILABLE   AGE
zhsun-worker-us-east-2a   1         1         1         1           6h
zhsun-worker-us-east-2b   1         1         1         1           6h
zhsun-worker-us-east-2c   1         1         1         1           6h

$ oc delete machineset zhsun-worker-us-east-2c
machineset.cluster.k8s.io "zhsun-worker-us-east-2c" deleted

$ oc delete machineautoscaler autoscale-us-east-2c
machineautoscaler.autoscaling.openshift.io "autoscale-us-east-2c" deleted
^C

$ oc logs -f cluster-autoscaler-operator-8684bc9f5c-xnb57
I0123 10:00:30.193001       1 machineautoscaler_controller.go:102] Reconciling MachineAutoscaler openshift-cluster-api/autoscale-us-east-2c
E0123 10:00:30.197162       1 machineautoscaler_controller.go:123] Error getting target: machinesets.cluster.k8s.io "zhsun-worker-us-east-2c" not found

Expected results:
Machineautoscaler could be deleted.

Additional info: