Automatic merge from submit-queue (batch tested with PRs 53694, 53919). If you want to cherry-pick this change to another branch, please follow the instructions <a href="https://github.com/kubernetes/community/blob/master/contributors/devel/cherry-picks.md">here</a>.

fix controller manager crash issue on a manually created k8s cluster

**What this PR does / why we need it**:
fix controller manager crash issue on a manually created k8s cluster, it's due to availability set nil issue in azure loadbalancer

**Which issue this PR fixes** *(optional, in `fixes #<issue number>(, fixes #<issue_number>, ...)` format, will close that issue when PR gets merged)*: fixes #
In the testing of a manually created k8s cluster, I found controller manager on master would crash in current scenario:
1. Use acs-engine to set up k8s 1.7.7 cluster (it's with an availability set)
2. Manually add a node to the k8s cluster (without an availibity set in this VM)
3. Set up a service and schedule the pod onto this newly added node
4. controller manager would crash on master because although this k8s cluster has an availability set, the newly added node's `machine.AvailabilitySet` is nil which would cause controller manager crash

**Special notes for your reviewer**:
@brendanburns @karataliu @JiangtianLi 

**Release note**:

```
fix controller manager crash issue on a manually created k8s cluster
```
/sig azure